feat: add Telegram rich message support

[sudoskys]

Summary

• add a Bot API 10.1 Rich Message backend via richify() and InputRichMessage
• add a living Rich Message PRD plus project control-plane docs and ADR automation
• preserve current MessageEntity fields when splitting/clipping and add date_time serialization support
• add CI entries for Rich Message unit tests and live sendRichMessage contract validation when Telegram secrets are configured
• add playground/rich_message_case.py for manual Rich Message showcase sends from playground/t_longtext.md

Verification

• pdm run test-rich -> 15 tests OK
• pdm run test-live-rich -> 1 test OK against the real Telegram Bot API; Telegram returned Message.rich_message
• pdm run python playground/rich_message_case.py -> sent playground/t_longtext.md as a real Rich Message, message_id 69, block_count 35
• pdm run test -> 218 tests OK, 1 skipped
• make adr-lint -> OK (0 ADRs)
• make adr-index -> regenerated docs/adr/ACTIVE.md
• pdm build -> sdist and wheel built successfully

Compatibility

• convert() remains tuple[str, list[MessageEntity]]
• richify() is a parallel output backend, not a replacement for entity output
• core runtime dependency remains pyromark only; PyYAML is in the tests optional group for ADR tooling

Notes

• Rich Message PRs now require pdm run test-live-rich with TELEGRAM_BOT_TOKEN and TELEGRAM_CHAT_ID before PR creation.