Add existing-window tab placement for Browser Bridge

[dyss1992]

Summary

Adds an opt-in Browser Bridge tab placement policy:

• OPENCLI_TAB_PLACEMENT=existing-window
• Reuses an already-open normal Chrome profile window
• Creates regular tabs in that window
• Does not create OpenCLI-managed Chrome windows
• Does not create, move, or rename OpenCLI tab groups
• Fails clearly when the selected Browser Bridge profile has no normal Chrome window open

Default behavior remains unchanged (owned-container).

Notes

This is intended for workflows where each Chrome profile is already opened and managed externally, and automation should stay inside that profile window instead of creating separate OpenCLI windows or tab groups.

In existing-window mode, owned tabs are removed on release instead of being converted into reusable blank placeholders, so regular user windows do not accumulate blank tabs.

Tests

• npm run typecheck
• npx vitest run --project unit src/browser/daemon-client.test.ts src/cli.test.ts src/execution.test.ts
• cd extension && npm run typecheck && npm run build
• npx vitest run --project extension extension/src/background.test.ts
• npm run build
• npm test (458 files, 4957 tests passed, 1 skipped)