Add optional Xquik publisher and repair MCP process startup

[kriptoburak]

Summary:

• add an opt-in Xquik provider for the active async X platform
• support single posts and reply-chained threads through the documented Xquik API
• document explicit provider selection and keep the auth hook aligned with runtime routing
• remove duplicate changes to the deprecated legacy implementation

Independent repository repairs:

• stop external MCP child output from filling unread pipes and deadlocking the proxy
• detect an MCP child that exits during startup immediately instead of waiting for the full 30-second timeout
• remove stale unused imports from the proxy

Validation:

• 215 tests passed, 4 skipped
• Ruff check and format checks passed for every touched Python file
• Python compileall passed
• git diff and secret-shape checks passed

Xquik is an independent third-party service. Not affiliated with X Corp. "Twitter" and "X" are trademarks of X Corp.

[kriptoburak]

Consolidated repair published as verified SSH-signed commit 27e02fb.

The branch now contains exactly 1 commit on current upstream main.

Xquik integration:

• Adds explicit opt-in Xquik routing for single X posts and reply-chained threads.
• Uses the current public Xquik API contract and matching auth validation.
• Preserves the existing direct X provider as the default.
• Documents the provider, API key, connected account, and optional base URL.

Independent repository repairs:

• Prevents external MCP child output from filling unread pipes and deadlocking startup.
• Detects an MCP child that exits during startup immediately.
• Clears failed child state and reports the actual exit status.
• Removes stale proxy imports.
• Adds focused regression coverage.

Validation:

• Full suite: 215 passed, 4 skipped
• Focused repair suite: 42 passed
• Ruff check passed for every touched Python file
• Ruff format check passed for every touched Python file
• Python compileall passed
• Xquik configuration contract probe passed
• git diff check passed
• GitHub signature verified

No contributor action remains. The PR is open, clean, and mergeable, with maintainer review pending.