feat(sessions): expose bounded page metadata

[SarthakWade]

Summary

• preserve the ordered sessions array while adding fixed-shape URL, title, age, isolation, lifecycle, and truncation metadata
• read metadata without focus changes, agent-control activation, semantic snapshots, or page JavaScript on WebKit and Chromium
• sanitize HTTP(S) URLs, remove userinfo, bound all page-derived strings, and isolate per-session failures and close races
• publish typed nested result contracts in the TypeScript and Python SDKs and record the cross-engine decision

Security

• marks the command and every detail as untrusted page-derived content
• exposes no cookies, storage, credentials, authentication state, process IDs, native IDs, or filesystem paths
• queries engines after releasing shared state locks and returns controlled unavailable states without leaking errors

Validation

• pnpm test
• pnpm test:runtime
• pnpm test:npm
• Python SDK: 42 tests, mypy, and ruff
• pnpm --filter @headless/web lint
• pnpm --filter @headless/web build
• pnpm --filter @headless/app build
• Linux Chromium P2 E2E in a provisioned test image
• focused real WKWebView normal and isolated session metadata flow

The full local macOS E2E currently stops before this scenario in the existing start-page address HUD accessibility click (Headless has no accessible windows). The focused WKWebView G4 flow passes.

Closes #196

[yashranaway]

session.list stays additive at protocol 0.5. The name array is unchanged. Each detail is a fixed shape: name, isolated, ageMs, status, nullable http(s) URL and title, truncation flags, and untrustedContent: true.

URLs drop userinfo and anything that is not http/https. Title is capped at 1,000 bytes, URL at 8,192. The host lock is released before engine I/O. WebKit reads WKWebView on the main thread. Chromium uses Page.getNavigationHistory plus host navigation flags, not page JS. A session that errors stays unavailable instead of failing the whole list.

CI is green, including SDK generation. Stacks on #198.