feat(graceful-handoff): 2.0 — Stop hook auto-execute + pipeline staging fix [skip-docs]

Step 5 of v2.0 plan + critical pipeline fix.

Stop hook (hooks/scripts/stop-context-monitor.mjs):
- Estimates context usage from transcript size (chars/3.5 / window_size)
- At ≥70%, spawns handoff-pipeline.mjs --auto --no-push synchronously
- Reads context_window_size from payload (supports 1M windows)
- Lock file at <transcript_dir>/.handoff-lock-<session_id>
- Gracefully handles missing CLAUDE_PLUGIN_ROOT, missing transcript

Pipeline fix (scripts/handoff-pipeline.mjs):
- REMOVED `git add -A` (CLAUDE.md anti-pattern: scoops up unrelated WIP)
- Now stages ONLY artifact + REMEMBER.md/TODO.md if present
- New regression test 'pipeline never stages unrelated dirty files'

Tests: 7 stop-hook tests use stub pipeline (no real git operations);
11 pipeline tests including new regression for explicit staging.

plugins/graceful-handoff/tests/scripts/handoff-pipeline.test.mjs

@@ -97,11 +97,15 @@ test('--no-commit skips git operations even when dirty', async () => {
   rmSync(repo, { recursive: true, force: true });
 });
 
-test('idempotency: second --auto run on clean state with recent artifact is no-op', async () => {
+test('idempotency: second --auto run on clean tree with recent artifact is no-op', async () => {
   const repo = makeTempRepo();
   writeFileSync(join(repo, 'foo.txt'), 'change\n');
-  // First run: dirty, commits
+  // First run: dirty, writes artifact and commits ONLY the artifact (not foo.txt)
   await runPipeline(repo, ['--auto', '--non-interactive', '--no-push']);
+  // Clean up the unrelated dirty file so second run sees a CLEAN tree.
+  // The pipeline must NEVER auto-stage user's other dirty files (CLAUDE.md
+  // anti-pattern) — the test explicitly removes it to isolate idempotency.
+  rmSync(join(repo, 'foo.txt'));
   // Second run: clean tree, recent artifact exists → idempotent no-op
   const result = await runPipeline(repo, ['--auto', '--non-interactive', '--no-push']);
   const json = JSON.parse(result.stdout);
@@ -112,6 +116,28 @@ test('idempotency: second --auto run on clean state with recent artifact is no-o
   rmSync(repo, { recursive: true, force: true });
 });
 
+test('pipeline never stages unrelated dirty files (no git add -A regression)', async () => {
+  const repo = makeTempRepo();
+  // Two unrelated dirty files — pipeline should NOT commit them
+  writeFileSync(join(repo, 'unrelated-1.txt'), 'user work\n');
+  writeFileSync(join(repo, 'unrelated-2.md'), '# user notes\n');
+  await runPipeline(repo, ['--auto', '--non-interactive', '--no-push']);
+  // After commit, unrelated files must STILL be in working tree (not committed)
+  const { execFileSync } = await import('node:child_process');
+  const lastCommit = execFileSync('git', ['show', '--name-only', '--pretty=', 'HEAD'], {
+    cwd: repo, encoding: 'utf-8',
+  }).trim().split('\n').filter(Boolean);
+  assert.ok(!lastCommit.includes('unrelated-1.txt'), `unrelated-1.txt should NOT be in HEAD commit, got: ${lastCommit}`);
+  assert.ok(!lastCommit.includes('unrelated-2.md'), `unrelated-2.md should NOT be in HEAD commit, got: ${lastCommit}`);
+  // The artifact SHOULD be in HEAD
+  assert.ok(lastCommit.some(f => f.includes('NEXT-SESSION')), `artifact should be in HEAD, got: ${lastCommit}`);
+  // unrelated files still untracked
+  const status = execFileSync('git', ['status', '--porcelain'], { cwd: repo, encoding: 'utf-8' });
+  assert.match(status, /unrelated-1\.txt/);
+  assert.match(status, /unrelated-2\.md/);
+  rmSync(repo, { recursive: true, force: true });
+});
+
 test('detached HEAD is detected and reported (no commit attempted)', async () => {
   const repo = makeTempRepo();
   // Detach HEAD