Suppress internal title-generation Slack notifications

[koichiro]

Summary

A single user prompt can trigger an internal Codex title-generation request that codex-notify posts as another Slack conversation. Suppress the complete known title-generation envelope and its subsequent response in Hook normal mode, and filter the corresponding events in both log-tail publishers before they enter delivery.

The classifier matches the full instruction envelope with whitespace normalization and a non-empty embedded prompt. Ordinary title requests, introduced/quoted instructions, and JSON responses with a title field remain eligible for delivery. Hook suppression persists across invocations and resume events; the next regular prompt clears it. Existing threads are preserved, while startup/clear and explicit thread resets retain their behavior.

Security and compatibility

• Prevents unnecessary forwarding of internal instructions and embedded prompt content without changing outbound secret redaction or expanding Hook event disclosure.
• Hook debug mode retains existing visibility. Log-tail filtering also suppresses internal tool events when tool notifications are enabled; the monitor-start notification remains unchanged.
• Existing ambient-task suppression retains its thread-clearing behavior. Log session metadata is used to track suppression without changing existing publication keys.
• Detection intentionally covers only the known template. An exact user-pasted copy is indistinguishable; changed templates and a log monitor started after the internal prompt cannot be classified from title JSON alone. These limits are documented in README.
• No credentials, real Hook payloads, session logs, or screenshot/account details were added. Regression inputs are synthetic. No live Slack or Codex reproduction was performed.

Validation

• bundle exec rake — 272 runs, 1,466 assertions, 0 failures, 0 errors, 0 skips; line coverage 96.07% (1,709/1,779).
• git diff --check — passed.
• Regression coverage includes separate/same-session delivery, persisted suppression, resume/reset, stale-thread avoidance, debug behavior, quoted instructions, JSON title responses, both log publishers, session metadata, mixed records, malformed JSON, and internal initial prompts.

Fixes #88