refactor: extract TabSession for per-tab state isolation (v0.15.16.0) (#873)

* plan: batch command endpoint + multi-tab parallel execution for GStack Browser

* refactor: extract TabSession from BrowserManager for per-tab state

Move per-tab state (refMap, lastSnapshot, frame) into a new TabSession
class. BrowserManager delegates to the active TabSession via
getActiveSession(). Zero behavior change — all existing tests pass.

This is the foundation for the /batch endpoint: both /command and /batch
will use the same handler functions with TabSession, eliminating shared
state races during parallel tab execution.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: update handler signatures to use TabSession

Change handleReadCommand and handleSnapshot to take TabSession instead of
BrowserManager. Change handleWriteCommand to take both TabSession (per-tab
ops) and BrowserManager (global ops like viewport, headers, dialog).
handleMetaCommand keeps BrowserManager for tab management.

Tests use thin wrapper functions that bridge the old 3-arg call pattern to
the new signatures via bm.getActiveSession().

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: add POST /batch endpoint for parallel multi-tab execution

Execute multiple commands across tabs in a single HTTP request.
Commands targeting different tabs run concurrently via Promise.allSettled.
Commands targeting the same tab run sequentially within that group.

Features:
- Batch-safe command subset (text, goto, click, snapshot, screenshot, etc.)
- newtab/closetab as special commands within batch
- SSE streaming mode (stream: true) for partial results
- Per-command error isolation (one tab failing doesn't abort the batch)
- Max 50 commands per batch, soft batch-level timeout

A 143-page crawl drops from ~45 min (serial HTTP) to ~5 min (20 tabs
in parallel, batched commands).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* test: add batch endpoint integration tests

10 tests covering:
- Multi-tab parallel execution (goto + text on different tabs)
- Same-tab sequential ordering
- Per-command error isolation (one tab fails, others succeed)
- Page-scoped refs (snapshot refs are per-session, not global)
- Per-tab lastSnapshot (snapshot -D with independent baselines)
- getSession/getActiveSession API
- Batch-safe command subset validation
- closeTab via page.close preserves at-least-one-page invariant
- Parallel goto on 3 tabs simultaneously

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: harden codex-review E2E — extract SKILL.md section, bump maxTurns to 25

The test was copying the full 55KB/1075-line codex SKILL.md into the fixture,
requiring 8 Read calls just to consume it and exhausting the 15-turn budget
before reaching the actual codex review command. Now extracts only the
review-relevant section (~6KB/148 lines), reducing Read calls from 8 to 1.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: move batch endpoint plan into BROWSER.md as feature documentation

The batch endpoint is implemented — document it as an actual feature in
BROWSER.md (architecture, API shape, design decisions, usage pattern)
and remove the standalone plan file.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* chore: bump version and changelog (v0.15.16.0)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: gstack <ship@gstack.dev>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/skill-e2e-workflow.test.ts

@@ -467,8 +467,18 @@ describeIfSelected('Codex skill E2E', ['codex-review'], () => {
     run('git', ['add', 'user_controller.rb']);
     run('git', ['commit', '-m', 'add vulnerable controller']);
 
-    // Copy the codex skill file
-    fs.copyFileSync(path.join(ROOT, 'codex', 'SKILL.md'), path.join(codexDir, 'codex-SKILL.md'));
+    // Extract only the review-relevant section from codex SKILL.md (~120 lines vs 1075).
+    // Full SKILL.md is 55KB / ~14K tokens — takes 8 Read calls to consume, exhausting turns.
+    const full = fs.readFileSync(path.join(ROOT, 'codex', 'SKILL.md'), 'utf-8');
+    const startMarker = '# /codex — Multi-AI Second Opinion';
+    const endMarker = '## Plan File Review Report';
+    const start = full.indexOf(startMarker);
+    const end = full.indexOf(endMarker, start);
+    const reviewSection = full.slice(
+      start >= 0 ? start : 0,
+      end > start ? end : undefined,
+    );
+    fs.writeFileSync(path.join(codexDir, 'codex-SKILL.md'), reviewSection);
   });
 
   afterAll(() => {
@@ -485,11 +495,11 @@ describeIfSelected('Codex skill E2E', ['codex-review'], () => {
 
     const result = await runSkillTest({
       prompt: `You are in a git repo on branch feature/add-vuln with changes against main.
-Read codex-SKILL.md for the /codex skill instructions.
-Run /codex review to review the current diff against main.
+Read codex-SKILL.md for the /codex review instructions (it's short — ~120 lines).
+Follow those instructions to run codex review against the diff on this branch.
 Write the full output (including the GATE verdict) to ${codexDir}/codex-output.md`,
       workingDirectory: codexDir,
-      maxTurns: 15,
+      maxTurns: 25,
       timeout: 300_000,
       testName: 'codex-review',
       runId,