feat(notifications): Render Rocket Chat as event-typed attachments

[srtab]

Stacked on #1237 — please merge that one first; this PR's diff includes only the Piece 2+3 changes once #1237 is in.

Summary

Replaces the plain-text RC payload with class-based renderers keyed by EventType, mirroring the existing @register_channel pattern.

New package `daiv/notifications/channels/rocketchat_renderers/`

• `base.py` — `RocketChatRenderer` ABC + shared helpers (`_fmt_tokens`, `_fmt_cost`, `_fmt_duration`, `_usage_field`, `_cost_field`, `_link`) + color constants.
• `registry.py` — `@register_renderer` decorator + `get_renderer(event_type)` lookup.
• `job_finished.py`, `schedule_finished.py`, `job_batch_finished.py` — one renderer per terminal event.

Channel change (`rocketchat.py`)

• New `_build_payload(notification, delivery)` calls `get_renderer(event_type)` and renders to `(text, attachments)`.
• Falls back to `_compose_text` when no renderer is registered, so any new `EventType` ships before its renderer.

What the user sees now

✅ Agent run on acme/api succeeded
┌─ (green sidebar)
│ Agent run on acme/api succeeded                                  ← clickable
│ ┌─────────────────────┬──────────────────────┐
│ │ Trigger             │ Duration             │
│ │ Manual              │ 1m 24s               │
│ ├─────────────────────┼──────────────────────┤
│ │ Usage               │ Cost                 │
│ │ 12.4k in · 38.1k out│ \$0.21                │
│ └─────────────────────┴──────────────────────┘
│ DAIV · just now

Three colors: green (success), red (failure), yellow (partial batch). Batch rollups also include a per-repo `✓ / ✗` breakdown (capped at 8 with overflow marker).

Notes

• Missing data hides fields rather than rendering `—`: a notification with no token/cost data shows just Trigger + Duration.
• Top-level `text` is the OS-notification line (`✅ Subject`); structured detail lives in attachments. Body content is intentionally not duplicated.
• The existing `test_happy_path_posts_to_user_dm` was updated to assert the new payload shape; `test_unknown_event_type_falls_back_to_plain_text` covers the fallback.

Test plan

• New `test_rocketchat_renderers.py` (35 tests): registry lookup (incl. EventType-vs-str equality), helper formatters (parametrized over None / 0 / k-rounding / edge cases), each renderer × success/failure, batch × {all-green, all-red, partial-yellow}, repo breakdown truncation.
• Updated `test_rocketchat_channel.py`: happy path now asserts attachment shape; new test for unknown-event-type fallback.
• Full notifications suite: 185 tests passing.
• CI: `make test` + lint + ty.
• Manual smoke: send a real notification to a Rocket Chat DM and confirm rendering matches the mockup.