Document multi-agent tab soft-lock / ownership protocol for shared Chrome

[Trace824]

Summary

Multiple MCP agents on one Chrome hit TAB_NOT_OWNED and can contend on the same compose tab. The ownership model exists in product; operators need a first-class protocol doc (who owns which tab, how to yield, how to recover).

Repro

• Context: Two MCP clients registered as agents against one FSB extension.
• Steps:
  1. Agent A navigates / focuses a tab.
  2. Agent B calls navigate/click on the same tab.
• Observed: TAB_NOT_OWNED (and related codes in mcp README); compose UI can be disrupted if ownership is ignored.

Code pointers (verified against /workspace/FSB)

• mcp/README.md — tab-ownership enforcement; TAB_NOT_OWNED, TAB_INCOGNITO_NOT_SUPPORTED, TAB_OUT_OF_SCOPE (~L67, ~L421).
• extension/utils/agent-tab-resolver.js — ownership reject (~L106).
• extension/ai/tool-definitions.js — multi-agent notes on tools.

Proposed fix

Document soft-lock protocol: one writer per tab; claim via normal ownership; do not click compose on unowned tabs; recovery steps when TAB_NOT_OWNED is returned.

Acceptance / regression

• Docs section linked from mcp README.
• Matches actual error codes (no invented APIs).

Owner

docs

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Priority: P2 · Owner: docs