Add trusted extension installation

[0YHR0]

What changed

• add a strict versioned extensions.lock for built-in and third-party runtime extensions
• reject unlisted installed Entry Points before importing extension Python code
• validate loaded manifests against locked capability and risk declarations
• add wheel metadata/SHA-256 preflight, no-dependency installation, and JSONL receipts
• expose trust provenance through GET /api/v1/extensions
• document the same-process trust boundary in English and Chinese

Why

Runtime Extension API v0.1 proved that separately distributed scenarios work, but any installed
Entry Point was imported on startup. This adds the minimum operator-controlled trust boundary
needed before encouraging real extension installation.

Impact

Operators pin exact extension bytes and declarations, install them through
agentmesh-extension-install, and can inspect effective trust status. This does not claim package
signatures or process isolation; extensions remain trusted code in the API process.

Validation

• python -m ruff check .
• python -m pytest -q (431 passed, 28 skipped)
• python -m pytest --cov=agentmesh --cov-fail-under=80 (83.21%)
• built the AgentMesh wheel and verified it contains extensions/default.lock
• verified the published Daily Brief v0.1.0 wheel against the repository lock and GitHub digest