docs(ai-cli): note that green leaves no context breadcrumb

[aguil]

Docs-only follow-up to #125.

docs/ai-cli-context-statusline.md said where session breadcrumbs live but not
when they exist. #125 changed that: the statusline now persists a breadcrumb
only while a session is yellow or red, so during a normal green session
~/.cache/ai-context-alerts/<session_id>.json is simply absent — which reads as
a broken cache if you go looking for it.

Adds the lifecycle to the existing paragraph:

• Breadcrumbs exist only while yellow or red.
• Green writes nothing and deletes any existing file; absence is the green
  state, and the Stop hook short-circuits on a missing file.
• While yellow or red, a refresh that changes neither the level nor the
  whole-percent usage skips the rewrite, so the file's mtime can lag the
  statusline by design.

Each claim was checked against the merged code rather than from memory:

Claim	Source
Green deletes, else writes nothing	executable_ai-statusline green branch — rm -f "$state_path" guarded by prev_level != green
Hook short-circuits on absence	executable_context-threshold-stop.sh:21 — [ ! -f "$state_path" ] && exit 0
Skip when level + pct unchanged	the elif write gate comparing level / pct / new_ui_alerted
pct is whole-percent	executable_ai-statusline:53 — cut -d. -f1

No behavior change. pre-commit --all-files and both Neovim suites pass;
prettier was clean on the first pass.