P13-T4: Add stdio proxy mode for compatibility with existing MCP clients

[SoundBlaster]

Description

Implements P13-T4: stdio proxy mode for the persistent broker. Adds BrokerProxy.run() (replacing the stub from P13-T1) and two new CLI flags (--broker-connect, --broker-spawn) so existing MCP clients (Cursor, Claude, Codex) can transparently route through the broker with a one-line config change. The legacy direct mode is the default and is unaffected.

Changed areas:

• src/mcpbridge_wrapper/broker/proxy.py — full BrokerProxy implementation (bidirectional stdio ↔ Unix socket forwarding, auto-spawn support, connection timeout)
• src/mcpbridge_wrapper/__main__.py — _parse_broker_args() + broker-mode branch in main()
• tests/unit/test_broker_proxy.py — 15 new unit tests
• tests/unit/test_broker_stubs.py — updated stub test to reflect implemented class
• SPECS/ — PRD, validation report, archive, follow-up tasks

Validation report: SPECS/ARCHIVE/P13-T4_Add_stdio_proxy_mode/P13-T4_Validation_Report.md

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactoring
• CI/CD improvement

Quality Gates

• make test - All tests pass with ≥90% coverage (533 passed, 90.5% total coverage)
• make lint - No linting errors (ruff check src/ — all checks passed)
• make format - Code is properly formatted
• make typecheck - Type checking passes (mypy src/ — no issues in 18 source files)
• make doccheck - Documentation is synced with DocC (if docs changed)

Documentation Sync

• Documentation changes are synced with DocC catalog (or N/A) — no docs/ changes in this PR; docs update deferred to P13-T6

Testing

• Added/updated tests for new functionality (15 new unit tests in test_broker_proxy.py)
• All tests pass locally (533 passed, 0 failed)
• Manually tested the changes

Checklist

• Code follows the project's style guidelines
• Self-review completed
• Comments added for complex code
• Documentation updated (if needed) — deferred to P13-T6
• No new warnings generated
• PR title is descriptive