fix(app): link background subagents to their sessions - #47455

Summary

• Make background subagent rows in Session details real links to their child sessions.
• Make finished, failed, and cancelled subagent timeline notices link to the same child sessions.
• Reuse the existing session navigation so the parent tab is retained and Escape returns to it.
• Preserve native keyboard, modified-click, and middle-click behavior while leaving shell rows and notices noninteractive.

Fixes #47265

Validation

• App and Session UI typechecks; app E2E typecheck.
• App unit tests: 823 passed, 1 skipped.
• Session UI timeline unit tests: 60 passed.
• Focused browser regressions: 21 passed with no retries, covering Session details and completed timeline links, pointer and keyboard navigation, Ctrl/Cmd-click, middle-click, narrow layouts, child sessions absent from the session-list cache, parent return, and notice truncation in desktop/mobile LTR and RTL layouts.
• Visual comparison: all six notice layouts were pixel-identical before/after (0 changed pixels), so the change is interaction-only.
• Production entry benchmark: 9/9 passed on each ref across three serial samples per scenario. Cold-session medians were 458.3 → 457.9 ms first-correct and 539.9 → 543.3 ms stable. These small samples are diagnostic, not a statistically established performance change.

Requested by: @Brendonovich (Brendan via Slack)