v2.8.0 — Sub-agent UI polish

Sub-agent inline UI polish: agent identity in the header, braille spinner,
smart duration/cost formatters, path-aware tool argument truncation, inline
failed-error hint, and an aggregated expanded footer.

Added

• Agent identity in the header — each delegation row shows the agent icon
  (🔭 explore, 🧠 oracle, 📚 librarian, ⚡ general, 📋 reviewer; ▸ for YAML
  agents) and the bold agent name colored by status.
• Braille spinner — ⠋⠙⠹⠸⠼⠴⠦⠧⠇⠏ animates at 10 fps while a delegation
  is running, driven by a per-row setInterval at 100 ms. Frame is computed
  from Date.now() so parallel delegations stay visually in sync.
• Smart duration formatter (formatDuration) — progressive precision
  <1ms → 47ms → 3.2s → 2m 7s → 1h 12m. Applied to both header
  elapsed time and tool-timeline entries; replaces the previous flat
  (0.0s) rendering for sub-second tools.
• Smart cost formatter (formatCost) — progressive tiers <$0.001 →
  $0.004 → $0.420 → $1.23 → $1235. Threshold guards prevent
  rollover artefacts (e.g. 99.999 renders as $100, not $100.00).
• Inline failed-error hint — when status is failed, the collapsed header
  surfaces a one-line, red-colored hint extracted from the result text (leading
  Error: stripped, clamped to 60 chars with ellipsis) so the user can see
  the cause without expanding.
• Path-aware tool argument truncation (truncatePath) — progress-reporter
  argument summaries for path / filePath keys collapse the middle
  (…/sub-agents/render.ts) instead of tail-cutting the filename. Falls back
  to left-truncating the basename when even that overflows the budget.
• Between-calls last-tool indicator — while a delegation is between tool
  invocations, the header keeps the last finished tool visible as
  ◷ <name> in muted color so the row never goes silent while the elapsed
  counter ticks.
• Split coloring on the active tool — accent wrench icon, toolTitle
  tool name, muted arg summary; matches the convention used by
  _shared/call-render.ts for top-level tool calls.
• Expanded footer aggregate — below the assistant output the expanded view
  renders <model> · Tools: 12× read · 4× bash · $0.004 (muted). Tool mix
  is sorted by call count descending, ties broken alphabetically. Any of the
  three bits is omitted when the corresponding data is unavailable.
• src/sub-agents/format.ts — new module exporting SPINNER_FRAMES,
  SPINNER_TICK_MS, getSpinnerFrame, formatDuration, formatCost, and
  truncatePath. Pure functions, no theme dependency, independently tested.
• src/sub-agents/icons.ts — new module exporting SUB_AGENT_ICONS and
  the getAgentIcon(name) fallback helper (▸ for unknown/YAML agents).
  Single source of truth shared by the call-line renderer and the result
  renderer.
• 53 new sub-agent rendering tests (598 → 648 total) covering header bits,
  current/last tool display, error hint extraction, footer aggregate, spinner
  cycling, and all formatter edge cases.

Changed

• Status word dropped from the collapsed header — the spinner / ✓ / ✗
  / ⚠ glyph carries the meaning; the redundant completed/failed/etc.
  word is gone. Matches the convention used by _shared/stats-render.ts.
• Model name moved out of the collapsed header into the expanded footer
  aggregate to reduce header noise.
• Render loop tick raised from 1 s to 100 ms (SPINNER_TICK_MS) to drive
  the spinner animation and keep the elapsed counter smooth between tool
  events.
• register.ts uses getAgentIcon() from icons.ts instead of the inline
  SUB_AGENT_ICONS[name] ?? "▸" fallback, eliminating the duplicated literal.

Fixed

• formatDuration(999.6) previously rendered as 1000ms because rounding
  spilled past the seconds boundary. Now bucketed after rounding, producing
  1.0s.
• formatCost(99.999) previously rendered as $100.00 because
  .toFixed(2) rolled the display past the tier threshold. Threshold guards
  (0.9995 / 99.995) bucket boundary values into the correct tier.
• getSpinnerFrame(now) now clamps now ≥ 0 so negative timestamps (JS
  % returns negative remainders, which would index out-of-bounds and return
  undefined) always resolve to a real spinner frame.
• truncatePath(p, 0) previously returned the full string because
  slice(-0) equals slice(0). Now returns "" for non-positive budgets.