fix(agent): D1-D3 cleanups — dead code, N+1 query, unvalidated outcome enum

Fixes D1-D3 of plans/2026-07-11-nomos-agent-code-review.md:

- D1: deleted isTaskTool — defined, never called (dispatch already checks
  handleTaskTool's own `handled` return value).
- D2: recordTouched issued one SELECT per entity slug found in a tool call's
  args; batched into one `WHERE slug = ANY($1)` query. Verified live: a turn
  naming three separate entities recorded involves edges for all three via
  the single batched lookup.
- D3: complete_task's outcome had a declared enum (success|failure|partial)
  in its tool schema but nothing validated it — an out-of-enum value (model
  typo or a weaker model not respecting the schema) silently persisted as-is,
  with only "failure" special-cased (anything else became status='done'
  regardless of what the value actually said). Now validated in
  handleTaskTool: empty defaults to "success" (unchanged), a recognized value
  passes through, anything else defaults to "partial" (safer than silently
  treating an unrecognized value as success) with a warning logged. Verified
  live: instructed the agent to call complete_task with outcome="unclear" —
  persisted as outcome='partial', not the literal invalid string.

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

cmd/nomos/tasks.go

@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"fmt"
+	"log/slog"
 	"strings"
 )
 
@@ -136,15 +137,6 @@ func taskToolDefs() []toolDef {
 	}
 }
 
-func isTaskTool(name string) bool {
-	switch name {
-	case "set_goal", "propose_plan", "update_plan_step", "ask_operator", "complete_task":
-		return true
-	default:
-		return false
-	}
-}
-
 // toInt coerces a JSON tool-arg number (float64 after unmarshal) to int.
 func toInt(v any) int {
 	switch n := v.(type) {
@@ -248,8 +240,23 @@ func (a *agent) handleTaskTool(ctx context.Context, sessionID, name string, args
 	case "complete_task":
 		outcome, _ := args["outcome"].(string)
 		summary, _ := args["summary"].(string)
-		if outcome == "" {
-			outcome = "success"
+		switch outcome {
+		case "":
+			outcome = "success" // no outcome given at all — assume success, the common case
+		case "success", "failure", "partial":
+			// valid, use as-is
+		default:
+			// The tool schema declares an enum, but a weaker model (or a
+			// typo) can still send anything — an unrecognized value used to
+			// persist as-is, silently, with only "failure" special-cased
+			// (store.completeTask derives status='failed' from it; anything
+			// else became status='done' regardless of what the value
+			// actually said). Default to "partial" rather than silently
+			// treating an unrecognized value as "success" — safer to
+			// under-claim than over-claim a task's outcome.
+			slog.Warn("nomos: complete_task got an unrecognized outcome, defaulting to partial",
+				"session", sessionID, "outcome", outcome)
+			outcome = "partial"
 		}
 		if err := a.store.completeTask(ctx, sessionID, outcome, summary); err != nil {
 			return fmt.Sprintf("error completing task: %v", err), true