feat(tui): animate active subagent nodes

[ajzrva-sys]

Summary

Replace the static ⛓ N active-subagent marker with a compact linked-node animation.

• render one node per active subagent, connected with stable-width links
• animate each node through a staggered fill cycle every 320 ms
• color node phases using the active theme
• preserve the existing status-bar visibility and width-budget rules
• mount the interval only while the linked-node segment is visible and clean it up on unmount
• include a compact README animation demonstrating active counts from one through five

Verification

• npm run check — passed
  • 135 test files passed
  • 1,462 tests passed, 4 skipped
  • TypeScript typecheck passed
  • ESLint passed
• focused status-rule suite — 27 tests passed
• Prettier check for both changed files — passed
• git diff --check — passed
• independent Codex review — PASS after lifecycle/performance review

Visual review

The final 320 ms linked-node animation was previewed in Safari and approved before this update.

[ajzrva-sys]

Follow-up to the automated review on #76023: the suggested lifecycle coverage is included in this revision.

• added an Ink-rendered fake-timer lifecycle test
• verifies the initial and next 320 ms animation frames
• unmounts/cleans up and verifies clearInterval
• scopes the interval to the rendered linked-node segment, so hidden indicators do not tick

The visual design also evolved from the original glyph pulse into the approved linked-node animation.