fix(overlay): prevent flash on first hotkey press after long idle (#19)

* fix(overlay): prevent flash on first hotkey press after long idle

- Add POST_SHOW_QUIESCENCE_MS (2000ms) constant
- On Focused(false), when grace_ms falls between 1500ms and 2000ms
  (the window that fires Escape on the first WebView2 focus flap
  after a long idle), defer to the 150ms deferred-hide thread
  instead of sending Escape directly
- Reset focus_reassert_used on flap so a second flap-pair in the
  same window gets another re-assert attempt
- Keep existing immediate-Escape path for focus loss well after show
  (grace_ms >= 2000ms) so real outside-clicks still hide instantly

Build verified with `cargo build --bin Nex`.

Fixes the bug where pressing Ctrl+Space after the overlay has been
idle past the 5s warm-release window causes the overlay to flash
briefly then hide, requiring a second press to actually open it.

* docs: forbid AI agent names in commit messages

- Add "Commit Message Content" section to AGENTS.md
- Commit message bodies must contain NO Co-authored-by trailers
  referencing any bot or AI, NO bot signatures, NO mentions of AI
  assistants, and NO references to agent systems
- Applies to commit messages, tag messages, branch descriptions,
  and any other git-related text