[codex] add an allowlisted Missive channel

[rajivsinclair]

What changed

• adds Missive as a built-in Push channel using outbound REST polling
• accepts comments only from explicit conversation and user ID allowlists
• persists and deduplicates comments in a private local SQLite inbox, skips existing history on first startup, and clears rejected content before storage
• posts Markdown replies into the exact originating conversation without sending or drafting email
• enforces Missive's sustained request-rate budget, honors Retry-After, and splits replies at the documented 8,000-character limit
• adds configuration, doctor checks, secret-location protection, channel contract coverage, and operator documentation

Why

This keeps Push's small outbound-only gateway model while making a shared Missive conversation available to the existing Claude Code, Codex, or Pi runtime. It intentionally uses comments as commands and posts as visible replies so the integration cannot accidentally send customer email.

Closes #153.

Validation

• cargo fmt --all --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo build --locked
• cargo test --locked (338 unit tests plus integration/doc tests)
• mkdocs build --strict