feat(mcp): expose Codex identity metadata to tools

[richardgetz]

Summary

• add nested _meta.codex identity metadata (threadId, sessionId, cwd) to model-invoked MCP tool calls
• add the same identity envelope to direct app-server MCP tool calls while preserving legacy flat _meta.threadId
• cover the contract with core unit tests and an app-server MCP suite assertion

Verification

• PATH=/tmp/codex-tools/bin:$PATH UV_CACHE_DIR=/tmp/codex-uv-cache CARGO_HOME=/tmp/codex-cargo-home just fmt
• PATH=/tmp/codex-tools/bin:$PATH CARGO_HOME=/tmp/codex-cargo-home just test -p codex-core mcp_tool_call_codex_meta
• PATH=/tmp/codex-tools/bin:$PATH CARGO_HOME=/tmp/codex-cargo-home just test -p codex-app-server mcp_server_tool_call_returns_tool_result
• PATH=/tmp/codex-tools/bin:$PATH CARGO_HOME=/tmp/codex-cargo-home just fix -p codex-core
• Companion For Human local contract tests: npm test

Review

• Independent post-change review ran with two reviewers.
• First reviewer found no issues.
• Second reviewer found two P1 contract issues: shared Codex session ids could collapse descendant threads in For Human, and cwd could be empty when no single local environment was selected.
• Fixed both issues in the companion For Human local tree and Codex metadata cwd fallback.
• Follow-up independent review found no remaining issues.

Notes

• This keeps Codex generic: no For Human-specific code paths are added.
• The companion For Human changes remain local in /Users/rick/Github/for-human and should be committed in that repo separately.