feat: write/edit robustness — fuzzy patch applier + worktree checkpoints (v2.7.1)

#3 Fuzzy patch applier: new pure fuzzy-match.ts (locateMatch, exact→trim→
unicode-canon→Levenshtein≥0.66, refuse-on-ambiguous) wired into pending_changes
applyOne/rewindOne so local-model whitespace/unicode drift in old_string no
longer loses the edit.

#4 Worktree checkpoint + conversation-trim: checkpoints table + checkpoints.ts
(shadow-commit of tracked+untracked into refs/boocode/checkpoints, hooked into
the 3 external-agent dispatcher paths) + POST restore route (reset --hard +
clean -fd -> transcript trim -> backend-session reset) + "Restore to here" UI.

Built by 3 parallel agents; DB-integration testing caught a created_at
self-deletion bug. Coder suite 234 passing; server+coder build + web tsc clean.
Builds on v2.7.0-mit. openspec write-edit-robustness.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/coder/src/index.ts

@@ -25,6 +25,7 @@ import { setInferenceContext, clearInferenceContext } from './services/tools/inf
 import { registerMessageRoutes } from './routes/messages.js';
 import { registerSkillRoutes } from './routes/skills.js';
 import { registerPendingRoutes } from './routes/pending.js';
+import { registerCheckpointRoutes } from './routes/checkpoints.js';
 import { registerAgentSessionRoutes } from './routes/agent-sessions.js';
 import { registerTaskRoutes } from './routes/tasks.js';
 import { registerInboxRoutes } from './routes/inbox.js';
@@ -214,6 +215,7 @@ async function main() {
   registerMessageRoutes(app, sql, broker, inferenceApi);
   registerSkillRoutes(app, sql, broker, inferenceApi);
   registerPendingRoutes(app, sql);
+  registerCheckpointRoutes(app, sql);
   registerAgentSessionRoutes(app, sql);
   registerTaskRoutes(app, sql, inferenceApi);
   registerInboxRoutes(app, sql);