fix(ui): unify FleetView selection markers with codebase ●/○ language

[xz-dev]

Closes: #122

Summary

FleetView's selection markers are outliers in the extension's own visual language. Everywhere else, state is shown with a plain filled/hollow circle pair; only FleetView reaches for exotic glyphs:

Location	filled	hollow
conversation-viewer.ts	● running	○ idle
agent-widget.ts	● active	○ inactive
index.ts agent list	• project	◦ global
fleet-list.ts (this)	⏺ (U+23FA)	◯ (U+25EF)

⏺ (BLACK CIRCLE FOR RECORD) and ◯ (LARGE CIRCLE) are niche, render inconsistently across terminal fonts, and ◯ is visibly oversized next to text. This PR aligns FleetView with the rest of the codebase: ● selected (accent) / ○ unselected (dim).

These are base geometric shapes (U+25CF/U+25CB) with the broadest terminal-font coverage. Selection is already carried by the accent-vs-dim color, so a compound glyph like ◉ (U+25C9 FISHEYE — outer ring + inner dot that muddies at small terminal sizes) adds noise over plain ●.

The change is one line (plus a doc comment and test expectations):

- return rosterIndex === sel ? theme.fg("accent", "⏺") : theme.fg("dim", "◯");
+ return rosterIndex === sel ? theme.fg("accent", "●") : theme.fg("dim", "○");

Relationship to #122

#122 identified the same glyph problem but proposed ◉/○ — introducing a third circle set instead of converging on the existing one — and bundled ~300 lines beyond the fix: a 220-line companion extension with its own /subagents-ui command and a 79-line shell script that sed -is the installed package under node_modules (breaks on every update, and its spinner patterns don't match the current source). The companion machinery is personal customization that belongs in user dotfiles, not the extension. This PR keeps only the core idea and points it at the codebase's established glyph pair.

Testing

npm run lint, npm run typecheck, npm run test all pass (731 tests).

Checklist

• npm run lint / typecheck / test pass
• Single-line behavioral change; no API changes
• No breaking changes

[xz-dev]