Replace blocking handoffs with advisory attention notices

[wolfiesch]

Why

Human assistance should use chat for coordination. The extension reminder must not prevent an agent from inspecting the page, doing other work, or finishing its task while it waits for a reply.

Changes

• Replace blocking handoff state with task-owned attention notices and request, status, resolve, and dismiss operations.
• Remove handoff-based command and cleanup gates, including restart restoration paths.
• Replace the required completion button with optional Open tab and Dismiss controls. Dismissal and expiry do not report successful completion.
• Preserve notice ownership, stale-ID isolation, explicit Pause, sensitive-field protections, and configured Commit review.
• Queue tab-local capture cleanup behind admitted same-tab work and preserve existing permissive-mode and credential-broker settings.
• Update schemas, SDKs, adapters, migration coverage, and active documentation together.

Verification

Passed on this branch:

• bun run workspace:typecheck
• bun run --cwd packages/sdk-typescript build
• bun test packages/extension/test packages/omp/test packages/mcp/test packages/sdk-typescript/test packages/gpt-control-driver/test
• bun run extension:build
• cargo test -p agenttab-protocol -p agenttab-host from host-rs
• Python SDK tests and tests/architecture/verify_protocol_schemas.py
• Browser-driven built-popup fixture covering the attention notice, Dismiss, explicit Pause, and Developer-mode revocation.

This PR remains draft pending the real-profile activation check.¹

Footnotes

1. The updated extension has not yet been activated in the real Chrome profile. Fixture checks do not establish end-to-end live-profile correctness. Remaining verification is to load the matching bundle, create a notice, exercise commands while it stays open, resolve it after inspecting the completed page condition, and finish the task without popup acknowledgement. ↩