fix(mobile): stabilize subagent card activity

[iscekic]

Summary

Keeps every subagent card at a stable three-row content height while permanently showing the subagent name, task name, and latest child-session tool activity for the full parent task lifecycle (pending/running/completed/error).

• Derives a small immutable presentation model (getChildSessionCardState) from the parent task tool part and child-session messages.
• The selector scans child assistant messages newest-first and returns the latest tool part regardless of status, so status transitions never blank the activity row.
• The card always renders all three one-line rows (numberOfLines={1}), only the text changes — never the structure or height.
• Preserves existing border, status badge, icons, press behavior, accessibility role, and transcript navigation, including for an errored parent task when its metadata still has sessionId.

Feature-state coverage

State	Coverage
Happy	Selector tests cover newest-tool selection and completed/error retention of the third row
Empty	Selector test covers pending/empty input+messages and fixed fallback copy (Subagent / Task / Waiting for activity); CTA is structurally absent (no transcript destination yet)
Unhappy, retryable	Not applicable — this card performs no fetch/mutation; child transcript hydration retry is owned by the existing sheet (unchanged)
Unhappy, non-retryable	Selector test proves an errored child tool remains the latest activity; task-session extraction test covers parent-error metadata retaining navigation when sessionId exists

Testing

• apps/mobile: pnpm format, pnpm typecheck, pnpm lint, pnpm check:unused, pnpm test (861 tests passing), pnpm format:check — all green on latest head.
• git diff --check clean from repo root.

E2E note

On-device Maestro verification hit a login/preflight boundary in this worktree's simulator session (the email OTP request/verify flow did not progress past the sign-in screen despite the backend OTP/token endpoints themselves responding successfully in earlier passes in this same session, and despite a validated Metro/API provenance preflight). This is classified as a test-environment/login blocker rather than a product failure: no acceptance-flow behavior was exercised or found failing. Automated coverage (selector + task-session extraction tests) covers every applicable feature state. Documenting per the accepted E2E exception process.