feat(subagents): default tree to active agents

[clabby]

Summary

• open the subagent tree in the active-agents view by default
• require f to show all agents and center the camera on the oldest matching agent when opening the tree or changing filters
• promote active descendants of inactive agents to visible roots
• show the current active or all filter beside the f footer helper

Testing

• cargo test -p tact tui::components::subagents::tests
• just check-fmt
• cargo check

Stack

Depends on #189.

[codspeed-hq]

Merging this PR will not alter performance

✅ 18 untouched benchmarks
⏩ 31 skipped benchmarks¹

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_{Comparing cl/agent-view-clean (2ae97ac) with cl/lock-fix (8d8d1af)}

Footnotes

1. 31 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩