feat: Phase 8 — nomos packages extracted into internal/nomos/{turngate,retrycap,messagequeue,assent,session}

Mechanical extraction of nomos internal components into plain-Go subpackages
per the hexagonal plan (ADR 0016 §3.1 rule 3):
  turngate/   — per-session turn serialization (plan 2026-08-03 F1)
  retrycap/   — per-turn run retry cap (maxRunRetries=3)
  messagequeue/ — operator-message queue for busy-turn re-entry (F2)
  assent/     — chat-assent detection (isAssent, isTypedConfirmation,
                ExtractPendingApprovals), decoupled from agent via
                []string input instead of persistedCall
  session/    — store (chat sessions, plan execution, DB persistence),
                migration runner + local emitEvent to break adapter
                dependency

internal/migrate/ — shared migration runner extracted from postgres pool,
                    used by both the oikos postgres adapter and session tests.

session package export-rename finishing touches remain; the four smaller
packages compile with passing tests. Depguard rules and ADR-0016 leaf-note
update deferred to a followup. VERSION 0.35.1.

internal/nomos/retrycap/retrycap.go

@@ -0,0 +1,170 @@
+package retrycap
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"strings"
+	"sync"
+)
+
+// MaxRunRetries is the per-turn cap on identical failing `run` tool calls.
+// After this many Failures with the same (target, command) key, the agent
+// loop refuses to dispatch the call again and instead surfaces a directive
+// to investigate *why* (ps/strace/lsof) or escalate to the operator.
+//
+// Background: session 1e9c7691 (2026-07-18) retried the same
+// `chown :10000 /mnt/media_local && chmod 2775 …` ~20 times across direct
+// runs, SSH-hop-via-hubris, wrapping in a shell script, and bare `echo test`
+// sanity checks. Each retry piled up another zombie process on the target
+// (knfsd was holding a kernel lock on the exported directory). The agent
+// only investigated *why* after the operator explicitly asked
+// "the command just keeps running?" — see
+// plans/2026-07-18-session-review-three-sessions.md P0.1.
+const MaxRunRetries = 3
+
+// RunRetryTracker deduplicates failing `run` calls within a single chat
+// turn (chatWith invocation). It is NOT persisted across turns — the cap
+// is per-turn, so a fresh turn after the operator responds can retry once
+// more. The intent is to break a tight retry loop within one turn, not to
+// permanently block the agent from ever attempting the operation again.
+//
+// Threading: the agent loop is single-goroutine per turn, but the tracker
+// is guarded by a mutex so future callers (e.g. concurrent tool dispatch)
+// stay safe. The mutex is uncontended on the current hot path.
+type RunRetryTracker struct {
+	mu     sync.Mutex
+	counts map[string]int
+}
+
+func New() *RunRetryTracker {
+	return &RunRetryTracker{counts: make(map[string]int)}
+}
+
+// RunFailureKey is the dedup key for "this is the same command against the
+// same target." Whitespace is collapsed so trivial reformatting
+// (newlines vs spaces, trailing whitespace) doesn't escape the cap. The
+// purpose field is intentionally NOT part of the key: the agent often
+// rephrases purpose between retries while issuing the same command.
+func RunFailureKey(target, command string) string {
+	collapsed := strings.Join(strings.Fields(command), " ")
+	target = strings.TrimSpace(target)
+	h := sha256.Sum256([]byte(target + "\x00" + collapsed))
+	return hex.EncodeToString(h[:])
+}
+
+// RecordFailure increments the failure count for the given key and returns
+// the new count. The caller should check `count > MaxRunRetries` BEFORE
+// dispatching to decide whether to skip the call.
+func (r *RunRetryTracker) RecordFailure(key string) int {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.counts[key]++
+	return r.counts[key]
+}
+
+// Failures returns the current failure count for a key (0 if unseen).
+func (r *RunRetryTracker) Failures(key string) int {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.counts[key]
+}
+
+// IsRunFailure reports whether a `run` tool call's outcome should count
+// as a failure for retry-cap purposes. A call counts as failed when:
+//   - the dispatch itself errored (callErr != nil), OR
+//   - the result text starts with "run on <target>: ERROR" — the
+//     shape classifyAndGate/sshExec produce when SSH or the command fails.
+//
+// Approvals queued ("requires approval") do NOT count as Failures: they
+// are pending operator action, not a command execution failure. A read
+// of the existing code paths (classifyAndGate in internal/mcp/server.go)
+// confirms the "ERROR" prefix is the stable failure signature for `run`.
+//
+// The resultText parameter is the MCP tool's RAW text result (not JSON-
+// re-encoded): when classifyAndGate returns a textResult like
+// "run on host:strong: ERROR ...", the MCP client unwraps it back to a
+// plain Go string (see mcpClient.callTool). The caller should pass that
+// raw string, not json.Marshal's output (which would quote-wrap it).
+func IsRunFailure(toolName string, resultText string, callErr error) bool {
+	if callErr != nil {
+		return true
+	}
+	if toolName != "run" {
+		return false
+	}
+	// "run on host:strong: ERROR ..." or "run on lxc:caddy: ERROR ..."
+	// Both shapes start with "run on ".
+	if !strings.HasPrefix(resultText, "run on ") {
+		return false
+	}
+	return strings.Contains(resultText, ": ERROR")
+}
+
+// RunResultText extracts the raw text from a `run` tool's result value as
+// returned by mcpClient.callTool — typically a Go string, but may also be
+// a []string (multi-content result) or other JSON-decoded shape. Returns
+// "" for shapes we don't recognize. Used by the retry-cap path so
+// IsRunFailure receives the un-quoted text form (see its doc comment).
+func RunResultText(result any) string {
+	switch v := result.(type) {
+	case string:
+		return v
+	case []string:
+		if len(v) > 0 {
+			return v[0]
+		}
+	case []any:
+		var b strings.Builder
+		for _, e := range v {
+			if s, ok := e.(string); ok {
+				b.WriteString(s)
+			}
+		}
+		return b.String()
+	}
+	return ""
+}
+
+// RunRetryDirective is the synthetic tool result returned to the model
+// when the retry cap is hit, in place of dispatching the call again. It
+// directs the agent to investigate *why* the command keeps failing before
+// retrying, or to surface the blocker to the operator.
+func RunRetryDirective(target, command string, Failures int) string {
+	return "Refused: this `run` against " + target + " has failed " +
+		itoa(Failures) + " times this turn — retry cap hit. The command:\n  " +
+		command + "\nis almost certainly blocked by something on the target " +
+		"(a hung process, a kernel lock, an unexported FS, a stuck SSH " +
+		"session, …) — NOT a transient gateway issue. Do NOT retry with " +
+		"different routing or quoting. Instead, BEFORE calling `run` again, " +
+		"investigate *why* the command hangs: e.g. `ps aux | grep <cmd>`, " +
+		"`lsof <path>`, `strace -f -p <pid>` or `strace -f <cmd>`, " +
+		"`mount | grep <path>`, `dmesg | tail`. If you find a structural " +
+		"blocker (e.g. a kernel lock on an exported NFS directory → " +
+		"unexport → mutate → re-export), say so to the operator and fix it " +
+		"with a different command. If you genuinely cannot diagnose, " +
+		"surface the blocker to the operator with what you've tried — do " +
+		"not just retry the same command."
+}
+
+// itoa is a tiny strconv.Itoa to keep this file dependency-free.
+func itoa(n int) string {
+	if n == 0 {
+		return "0"
+	}
+	neg := n < 0
+	if neg {
+		n = -n
+	}
+	var buf [20]byte
+	i := len(buf)
+	for n > 0 {
+		i--
+		buf[i] = byte('0' + n%10)
+		n /= 10
+	}
+	if neg {
+		i--
+		buf[i] = '-'
+	}
+	return string(buf[i:])
+}