perf(agent): cache the MCP tool list per client (F1)

Fix F1 of plans/2026-07-11-nomos-agent-code-review.md, the last item.
buildTools called listToolsFull (a tools/list MCP round-trip) at the start of
EVERY chat turn, including every auto-continuation resume — the tool list is
static for the lifetime of one MCP connection, changing only when the api
process re-registers tools (a restart, which this client already detects and
reacts to via reconnectLocked). Re-fetching it every single turn was
avoidable network+parsing work on the hot path.

mcpClient now caches the parsed tool list after its first fetch, guarded by
its own mutex (kept separate from the request-serializing mu so a cache
check never contends with an in-flight doRequest call). reconnectLocked
clears the cache — an api restart may have changed what's registered, so a
stale cache would be wrong, not just slow. fleetSnapshot's get_health_summary
call is deliberately left uncached — it's meant to be "as of now."

Since each session gets its own client (the per-session pool from the
concurrency work), this caches per-task-conversation rather than globally: a
task's FIRST turn still pays the round-trip, every turn after reuses the
cached list — which is exactly the case that mattered (long-running,
heavily-autonomous tasks with many auto-continuation resumes).

Verified live via the api's request log: a brand-new session's first turn
made 3 MCP calls (initialize, tools/list, get_health_summary); a second turn
on the SAME session made exactly 1 (only get_health_summary) — tools/list
correctly skipped.

This completes the implementation order in
plans/2026-07-11-nomos-agent-code-review.md — every A/B/D/E/F finding from
the review (excluding C1, explicitly deferred per operator instruction) is
now fixed, tested, and verified live.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cmd/nomos/agent.go

@@ -721,7 +721,23 @@ func (a *agent) buildTools(sessionID string) ([]openai.ChatCompletionToolParam,
 	return tools, nil
 }
 
+// listToolsFull returns the MCP server's tool list, cached on this client
+// after the first call (see mcpClient.toolsCache). Fix F1 of
+// plans/2026-07-11-nomos-agent-code-review.md: buildTools calls this at the
+// start of every chat turn, including every auto-continuation resume — the
+// tool list is static for the lifetime of one MCP connection, so re-fetching
+// it every single time was avoidable network+parsing work on the hot path.
+// Cache invalidates on reconnectLocked (an api restart may change what's
+// registered).
 func (c *mcpClient) listToolsFull() ([]toolDef, error) {
+	c.toolsMu.Lock()
+	if c.toolsCache != nil {
+		cached := c.toolsCache
+		c.toolsMu.Unlock()
+		return cached, nil
+	}
+	c.toolsMu.Unlock()
+
 	resp, err := c.doRequest("tools/list", map[string]any{})
 	if err != nil {
 		return nil, err
@@ -744,5 +760,9 @@ func (c *mcpClient) listToolsFull() ([]toolDef, error) {
 			InputSchema: t.InputSchema,
 		}
 	}
+
+	c.toolsMu.Lock()
+	c.toolsCache = out
+	c.toolsMu.Unlock()
 	return out, nil
 }