fix(coder): harden edit-apply pipeline against block duplication

Root cause: two proven corruption mechanisms — (M1) non-idempotent apply
stamped the same block N times when a quantized model re-emitted the same
edit_file call or a turn was retried; (M2) Levenshtein tier 4 was fail-open
with no uniqueness guard, silently splicing into the wrong location.

Fixes applied at every layer of the pipeline:

Matcher (fuzzy-match.ts): raise SIMILARITY_THRESHOLD 0.66 → 0.85; add
AMBIGUITY_EPSILON uniqueness guard — two windows within 0.05 of the top
score → ambiguous, not a guess; add block-anchor gate (≥3-line needles
require first+last line exact match before a window is scored).

Edit planner (pending_changes.ts): extract planEdit() as a pure function;
idempotency guards detect already-applied states (anchored insert re-stamp,
old-gone-but-new-present); findPendingDuplicate() collapses identical
pending rows at queue time so M1 never reaches applyOne.

Atomic writes (pending_changes.ts): temp-file + rename on the same
filesystem so a crash can't leave a half-written source file; realpath()
first so symlinks survive the rename.

Per-file mutex (pending_changes.ts): withFileLock() serializes concurrent
read-modify-write on the same path via a chained-Promise Map.

EOL preservation (pending_changes.ts): normalize CRLF → LF for matching,
restore native line ending on write so Windows-style files stay clean.

Context isolation (inference_context.ts): replace module-level singleton
with AsyncLocalStorage so concurrent inference runs (arena parallel
dispatch, dispatcher poll racing a user message) each get their own
scoped context with no clobbering.

Tests: plan-edit.test.ts (pure planEdit unit tests), extended fuzzy-match
and pending_changes_integration suites, ALS isolation test that proves
overlapping runs get correct session IDs.

apps/coder/src/services/tools/apply_pending.ts

@@ -26,6 +26,15 @@ export const applyPendingTool: ToolDef<ApplyPendingInputT> = {
     },
   },
   async execute(_input: ApplyPendingInputT, projectRoot: string, context: ToolContext): Promise<unknown> {
+    // Under Ask (and Plan) the human approves via the Pending Changes panel — the
+    // agent must not auto-apply. Bypass and legacy (undefined) may apply.
+    if (context.permissionMode === 'ask' || context.permissionMode === 'plan') {
+      return {
+        status: 'denied',
+        message:
+          'Permission mode is Ask — staged changes must be approved by the user in the Pending Changes panel, not applied by the agent.',
+      };
+    }
     const results = await applyAll(context.sql, context.sessionId, projectRoot);
     const succeeded = results.filter((r) => r.success).length;
     const failed = results.filter((r) => !r.success).length;