Sidebar tab awareness: tabs.json has empty url/title on non-localhost sites since v1.14 (manifest permission gap)

## Summary

Since `v1.14.0.0` (`ed1e4be2`), the live tab awareness feature ships with a manifest permission gap: the extension can't read `tab.url` / `tab.title` for any site that isn't `127.0.0.1`, so `tabs.json` is written with empty strings and `active-tab.json` stops updating after the first navigation away from `/welcome`.

Net effect: the sidebar Claude Code REPL can't see what tab the user is actually on, and falls back to stale state from initial launch.

## Repro

1. `gstack browse connect` — Chromium opens at `127.0.0.1:34567/welcome`
2. Manually navigate (URL bar or click a link) to any non-localhost page, e.g. `https://news.ycombinator.com`
3. Read state files:
```bash
cat /.gstack/tabs.json
cat /.gstack/active-tab.json
```

### Actual

`tabs.json` updates timestamp + `tabId`, but `url` and `title` are empty:

```json
{
"updatedAt": "2026-04-28T06:59:36.945Z",
"reason": "updated",
"tabs": [{
"tabId": 1927305561,
"url": "",
"title": "",
"active": true,
"windowId": 1927305560,
...
}]
}
```

`active-tab.json` is stale, frozen at the welcome page:

```json
{"tabId":1927305561,"url":"http://127.0.0.1:34567/welcome","title":"GStack Browser"}
```

The Side Panel REPL then reports the user is still on the welcome page, even though the visible Chromium window is on a different site.

### Expected

`tabs.json` and `active-tab.json` should reflect the current real URL / title.

(`browse status` from CLI works fine — the daemon sees the live URL — so this is isolated to the extension → state-file pipeline.)

## Root cause

Permissions, not code. The code path is correct end-to-end:

- `extension/background.js:521` `snapshotTabs()` calls `chrome.tabs.query({})` and uses `t.url || ''` as a fallback.
- `extension/background.js:571` `chrome.tabs.onUpdated` listener fires `pushTabState('updated')`.
- `browse/src/terminal-agent.ts:441` writes `tabs.json` atomically.
- `browse/src/terminal-agent.ts:457` writes `active-tab.json` only if `active.url` is truthy and not `chrome://` / `chrome-extension://`.

The break is at the Chrome boundary: with the current `manifest.json`,

```json
\"permissions\": [\"sidePanel\", \"storage\", \"activeTab\", \"scripting\"],
\"host_permissions\": [\"http://127.0.0.1:*/\", \"ws://127.0.0.1:*/\"]
```

Chrome MV3 returns `tab.url` / `tab.title` only when one of:
- `\"tabs\"` permission is granted (broad), or
- `host_permissions` matches the tab's URL, or
- `activeTab` is granted **and** the user just clicked the extension action on that tab (does not apply to background polling).

For any site outside `127.0.0.1`, none of these hold, so Chrome returns `undefined` for both fields. `snapshotTabs()`'s `|| ''` fallback then writes empty strings, and the active-tab gate (`if (active && active.url && ...)`) silently skips the write.

This means **the feature has never worked outside localhost** since it shipped — not a regression from a later version.

## Git timeline

- `7665adf4` (v0.12.0) — extension introduced; only listened to extension's own messaging, didn't read arbitrary tab URLs.
- `dc0bae82` (v0.12.6.0) — \"sidebar agent uses real tab URL instead of stale Playwright URL\" (related but different code path).
- `ed1e4be2` (v1.14.0.0) — added the `chrome.tabs.onUpdated` listener + `tabs.json` writer for live tab awareness, but the manifest diff only added `\"ws://127.0.0.1:*/\"` for the new PTY channel; tab-related permissions weren't broadened.

## Proposed fix

One line in `extension/manifest.json`. Two reasonable options:

**Option A (minimal scope, recommended)** — add `\"tabs\"` permission:

\`\`\`diff
- \"permissions\": [\"sidePanel\", \"storage\", \"activeTab\", \"scripting\"],
+ \"permissions\": [\"sidePanel\", \"storage\", \"activeTab\", \"scripting\", \"tabs\"],
\`\`\`

The `\"tabs\"` permission is a no-warning install-time grant in MV3 (it's a non-host permission). It exposes `tab.url` / `tab.title` / `tab.favIconUrl` to `chrome.tabs.query()` etc. across all tabs, but does **not** grant content-script injection or cross-origin fetch — so it's a tight scope match for what `snapshotTabs()` actually needs.

**Option B** — broaden host permissions:

\`\`\`diff
- \"host_permissions\": [\"http://127.0.0.1:*/\", \"ws://127.0.0.1:*/\"],
+ \"host_permissions\": [\"http://127.0.0.1:*/\", \"ws://127.0.0.1:*/\", \"\"],
\`\`\`

This works but triggers the install-time \"Read your browsing history on all websites\" warning. Heavier than necessary.

Recommend Option A.

## Test idea

Add an e2e test in the headed-mode harness that:
1. Navigates the Playwright-controlled Chromium to a non-localhost URL (e.g., `https://example.com`).
2. Reads `tabs.json` after a 1s settle.
3. Asserts `tabs[0].url === 'https://example.com/'` and `tabs[0].title` is non-empty.

Without the manifest fix, this test fails with empty strings — guards against the same regression on future MV3 changes.

## Environment

- gstack `v1.15.0.0` (also reproduces by inspection of v1.14.x manifest)
- macOS 25.4.0 (Darwin)
- Chromium launched via `gstack browse connect` (Playwright-managed, headed)

---

Found by reading state files + `extension/manifest.json` + `background.js` + `terminal-agent.ts` after a Side Panel session reported \"can't see current tab.\" Happy to PR if useful.

Start with extension/manifest.json and the snapshotTabs() and chrome.tabs.onUpdated paths in extension/background.js; review how browse/src/terminal-agent.ts writes tabs.json and active-tab.json. Verify non-localhost navigation produces the real URL and title, then run or add the headed-mode e2e check described in the issue and confirm both state files update correctly.

Written by the indexing model from the issue text.