feat(mcp): expose browser tools to all harnesses

[edwin-zvs]

Summary

• expose browser_open/browser_inspect/browser_screenshot/browser_eval through agentd-mcp so MCP-enabled harnesses can use browser tooling
• add a trusted session.emit_event IPC path so MCP browser tools can emit BrowserPreview events into the calling session
• inject agentd-mcp into headless Claude/Codex turns as well as existing interactive sessions

Validation

• cargo check
• cargo test -p agentd-mcp browser::tests -- --nocapture
• cargo build

Relevant binaries

• agentd-mcp: new MCP browser tools live here
• agentd: session.emit_event IPC path
• agentd-adapter-claude / agentd-adapter-codex: headless MCP injection updates
• agent: thumbnail overlay rendering is reused from main