fix(tui): restore subagent navigation

[aryaminus]

Issue for this PR

Closes #34457

Also addresses the cancelled subagent navigation part of #32432 and the nested child navigation part of #15972.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This restores navigation into subagent sessions after task cancellation or interruption.

The main regression was that failed task rows could have a valid child session id, but the TUI treated the failed state as an error-expansion click and returned before running the row click handler. This PR lets failed rows with an onClick navigate, while preserving error expansion for failed rows that do not navigate anywhere.

It also keeps task cards linkable when the child session id is on top-level part metadata, keeps the subagent footer hint visible for completed/cancelled subagents, and fixes session.child.first to choose true direct children instead of siblings in nested sessions.

This does not change Windows-specific ctrl+x down key handling from #33085 or remap Ctrl+X from #23322.

How did you verify your code works?

• npx --yes bun@1.3.14 test src/components/message-part.test.ts from packages/session-ui
• npx --yes bun@1.3.14 run test test/cli/run/footer.view.test.tsx from packages/opencode
• npx --yes bun@1.3.14 run test test/cli/tui/inline-tool-wrap-snapshot.test.tsx from packages/tui
• npx --yes bun@1.3.14 typecheck from packages/session-ui
• npx --yes bun@1.3.14 typecheck from packages/opencode
• npx --yes bun@1.3.14 typecheck from packages/tui
• pre-push hook: bun turbo typecheck (30 successful, 30 total)

Screenshots / recordings

No screenshot; this is a navigation/click behavior fix.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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