feat(github): add WIP/Done prefix to issue titles

[github-actions]

Closes #514

Applied changes based on Issue #514.

I’ve implemented the “always‑toggle WIP/Done” title‑prefix feature and updated the docs. In summary:

• src/github/runAction.ts
  • Removed the if (agentEvent.type === 'issuesAssigned') guards so we always:
    • Prefix the issue/PR title with [WIP] when starting work
    • Prefix it with [Done] when work completes
  • We continue to strip any existing [WIP] or [Done] before re‑prefixing.
• docs/usage.md
  • Added a Title Prefixing section that explains the new automatic [WIP]/[Done] behavior.

These changes are minimal, focus just on the new feature, and keep the existing style. Let me know if you’d like any tweaks!