feat: add bounded App Store Connect Code Mode

[keithah]

Summary

• add opt-in --code-mode exposing three bounded catalog and execution tools for MCP clients that cannot load all 502 domain tools
• route bridged calls through the existing dispatcher and preserve read-only/mutation policy gates
• update documentation, changelog, contributor guidance, and stdio smoke coverage

Verification

• swift test — 1,870 tests in 139 suites passed
• swift build -c release passed
• default release-binary MCP stdio smoke — 502 tools passed
• Code Mode release-binary MCP stdio smoke — 3 tools and structured error behavior passed
• git diff --check passed