fix(sessions): wire finish, retry and usage signals to OpenCode V2 events (#1033)

## What changes for the user
Four things that silently stopped working with OpenCode V2 work again:

- **"A session finished" notifications** arrive again. A successful or
failed run notifies. A run you stopped yourself does not.
- **Provider retries** show the retrying status and the countdown toast
again.
- **Usage statistics** refresh after each run.
- **The token usage line** under assistant replies (preference,
transcript filter row and hotkey) shows each settled reply's tokens and
cost again.

Per-session transcript reducers are also released again once a run has
settled.

## Why
OpenCode V2 declares `session.idle` and `session.status` in its schema,
but core never publishes them. Runs end with
`session.execution.{succeeded,failed,interrupted}`, and retries arrive
as `session.retry.scheduled`. These features were still keyed only to
the V1 events.

The token usage line had a second problem: it was drawn from V1
`step-finish` parts, which V2 never produces. V2 stores tokens and cost
on each assistant message instead.

A new isolated native fixture,
`scripts/test-session-lifecycle-native.mjs`, confirms this on CLI 2.1.0.
It uses its own storage, port and fake provider, and never the shared
service. In its complete, retry, fail and interrupt scenarios, it sees
no `session.idle` or `session.status` event. Upstream 2.0.27 sources
show the same: these events are declared but never published.

## How
- **Finish notification:** sent from the V2 terminal path after
`session.active` confirms the run ended.
  - Succeeded and failed runs notify.
- User and shutdown interrupts stay silent; idle-timeout interrupts
notify.
  - Subsessions and unknown sessions stay silent.
  - A stray `session.idle` cannot cause a duplicate.
- **Retry:** `session.retry.scheduled` sets the retry state and the
transient toast. The next attempt or the end of the run clears them.
- **Run-end detection:** a shared `lib/native-execution-events.ts`
drives the usage refresh and reducer retirement.
- **Reducer retirement:** it also waits for queued prompts, so prompts
left in the inbox after an interrupt stay listed.
- **Usage line:** moves to `components/message-usage.tsx` with the
existing classes.
- **Removed:**
  - the dead V1 `StepCard`, its step-part types and CSS;
  - the V1 `session.idle`/`session.status` handlers.
- **AGENTS.md:** one bullet so future features wire to V2 lifecycle
events.

Left for the follow-up cleanup PR: the remaining dead `session.idle`
fallbacks in pending discovery, Missions triggers and test fixtures.

## Validation
New tests that fail on unmodified dev and pass here:

| Suite | dev | this PR |
|---|---|---|
| lifecycle unit | 3/4 fail | 4/4 |
| retirement | 8/9 fail | 9/9 |
| browser `usage-metrics` | 3/3 fail | 3/3 |
| browser `service-usage` | 2/3 fail | 3/3 |

- UI typecheck is clean.
- **Passing unit suites:**
- opencode-data-idle 9, runtime-contract 4, session-native-events 8,
message-part-grouping 6
- opencode-transcript-replay 17, opencode-compaction 11,
session-pruning-projection 2, session-pruning-pagination 6
- session-request-authority 77, opencode-data 30, message-timeline-v2 2,
transcript-visibility 8
  - missions 1, instances-pending-compaction 7
- **Passing browser suite:** `system-messages` 10/10.
- The native lifecycle capture passes.

Oversized files touched:

| File | Lines |
|---|---|
| `message-block.tsx` | ~1,760 (down ~240) |
| `message-section.tsx` | ~1,630 |
| `opencode-data.ts` | ~905 |
| `session-events.ts` | ~670 |
| `message-timeline.tsx` | ~645 |
| `message-base.css` | ~675 |
| `session-request-authority.test.ts` | ~2,400 |
| `opencode-data.test.ts` | ~1,215 |

dev-docs/architecture.md

@@ -61,7 +61,7 @@ Yolo state endpoints currently live at `/workspaces/:id/yolo/sessions/:sessionId
 
 The server holds one `client.event.subscribe()` stream. `InstanceEventBridge` maps native location events to CodeNomad `instance.event` records, and `/api/events` multiplexes them with workspace and Yolo events for the browser; heartbeats use `POST /api/client-connections/pong`. The stream is volatile and has no replay guarantee: reconnect must reconcile sessions and pending requests and reread authoritative file/config state. Pending recovery uses the loaded-only broker, not worktree-wide ordinary lists; see [compatibility](OPENCODE_V2_COMPATIBILITY.md). Relay ordering and ownership fences remain specified in [AGENTS.md](../AGENTS.md).
 
-Current native events include session lifecycle/output events (`session.created`, `session.renamed`, `session.moved`, `session.status`, `session.idle`, `session.execution.*`, `session.compaction.*`, `session.text.*`, `session.reasoning.*`, `session.tool.*`), background Shell refresh via `shell.created`, `shell.exited`, `shell.deleted`, file invalidation via `filesystem.changed`, and configuration invalidation via `config.updated`.
+Current native events include session lifecycle/output events (`session.created`, `session.renamed`, `session.moved`, `session.execution.*`, `session.retry.scheduled`, `session.step.*`, `session.compaction.*`, `session.text.*`, `session.reasoning.*`, `session.tool.*`), background Shell refresh via `shell.created`, `shell.exited`, `shell.deleted`, file invalidation via `filesystem.changed`, and configuration invalidation via `config.updated`. Each busy period settles with exactly one `session.execution.succeeded`/`failed`/`interrupted`; the schema still declares `session.status` and `session.idle`, but OpenCode V2 never publishes them.
 
 ## Feature Ownership