docs(server): document provider turn startup recovery

[patroza]

What Changed

• add an operator runbook for diagnosing and recovering stuck provider turns
• document the global command-worker head-of-line failure mode and startup reconciliation behavior
• record the 2026-07-31 incident pattern, safe SQLite diagnostics, and normal/emergency systemd recovery
• define prevention priorities and acceptance tests for timeouts, per-thread isolation, sweeping, and readiness
• cross-link the runbook from the docs index and composer lifecycle analysis

Why

One wedged provider session start can currently block unrelated turn starts while the process and HTTP listener remain alive. Recovery is reliable only through service restart and startup replay. This documents the present operator path and the product work needed to make recovery automatic.

Downstream Fork Relationship

Base: fork/changes. Affected area: documentation only. No dependency on another open PR. Deployment-specific hostnames, credentials, monitoring, and restart automation remain assigned to the private operations repository.

Verification

• git diff --check
• vp check (zero errors; existing repository warnings only)
• ELECTRON_SKIP_BINARY_DOWNLOAD=1 vp run -r --cache --log labeled typecheck

Checklist

• This PR is small and focused
• I explained what changed and why
• UI screenshots are not applicable
• Animation/interaction video is not applicable