feat(ui): minimal Done/Failed push, skip for agent-continuation turns

[floodsung]

Summary

The post-task completion notice was repeating everything already in the card's grey footer:

✅ Done (52s · $340.41 · opus-4-7 · 377.3k/1000k (38%))

But the whole point of the notice is the mobile push — card edits don't ring on Feishu mobile, text messages do. Once the user opens the chat, the card footer already shows duration / cost / model / context usage. The push banner just needs to say "the thing you asked about finished."

Changes

• Trim notice body to `✅ Done` / `❌ Failed` — no parens, no stats. Card footer keeps the detail.
• Skip notice for continuation turns (`handleContinuationTurn`) — those are between-turn agent activity (background task return, teammate ping, `/goal` evaluator). The user never explicitly asked for them, so a mobile push is noise. The blue→green card remains as the visual signal in-chat.
• Preserved: 10-second threshold (quick tasks stay silent), `skipCompletionNotice` opt-out for WeChat (already routes final response as its own message), normal-turn notices on both happy path and stale-session retry.
• Side cleanup: `startTime` in `handleContinuationTurn` was only used to compute `durationMs` for the now-removed notice — both gone.

Tradeoff

A continuation turn that ends in error (e.g. background bash task crashed and the agent's summary turn surfaces the failure) no longer pushes. The user will see the red card next time they open Feishu. If this proves to be a real problem we can re-add a notice gated on `status === 'error'` for continuation turns specifically. Leaving it out for now — the user explicitly asked for the silence.

Test plan

• `npm run build` clean
• `npx vitest run` — 291 / 291 pass
• `npm run lint` — 0 errors, 2 pre-existing warnings (untouched)
• Manual: run a >10 s user-prompted task on Feishu, confirm push notification body is exactly `✅ Done`
• Manual: trigger a background-task continuation, confirm a card appears (blue → green) but no `✅ Done` text message follows
• Manual: trigger a failing user-prompted task, confirm push body is `❌ Failed`

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