feat: add compaction-counter widget

[CorticalCode]

Summary

• Adds a compaction-counter widget that tracks context compaction events per session
• When Claude Code compacts conversation context, used_percentage drops — this widget detects the drop and displays ↻N
• Addresses the detection gap noted in Ctx is not being updated after /compact #92 by using statusline data instead of JSONL transcripts

How It Works

Context only grows until compaction — any drop in used_percentage > 2 percentage points between consecutive renders is counted as a compaction event. The 2-point threshold filters rounding noise and cache accounting wobble (±1 point) while reliably catching real compaction events on both 200K and 1M context windows.

Per-session state is stored in ~/.cache/ccstatusline/compaction/compaction-{sessionId}.json, following the skills caching pattern.

Caveats

• Manual /compact detection is delayed until the next assistant message (no statusline update on /compact itself)
• used_percentage excludes system overhead (~15%, per context_window.used_percentage in status line JSON is significantly inaccurate anthropics/claude-code#34537), but relative drops between renders are still observable
• The 2% threshold is a reasonable starting point; real-world telemetry would help calibrate further

Changes

File	Change
src/utils/compaction.ts	New — detection logic + session state I/O (Zod-validated)
src/widgets/CompactionCounter.ts	New — widget class (returns ↻N or null, supports preview mode)
src/utils/__tests__/compaction.test.ts	New — 11 detection unit tests
src/widgets/__tests__/CompactionCounter.test.ts	New — 13 widget tests
src/types/RenderContext.ts	Added CompactionData interface and compactionData field
src/ccstatusline.ts	Compaction detection in render pipeline (conditional, only when widget is configured)
src/utils/widget-manifest.ts	Registered compaction-counter
src/widgets/index.ts	Barrel export

Design Decisions

• No configurable threshold — kept simple for first contribution. 2% handles both 200K and 1M windows.
• Conditional gathering — compaction detection only runs when the widget is present in the configured layout (follows existing pattern for speed/skills widgets)
• Zod validation on state file parse to handle corruption gracefully
• JSON state files (not JSONL) since we only track 2 numbers per session

Test Plan

• bun test — 24 new tests pass (11 detection + 13 widget), no regressions
• bun run lint — zero errors, zero warnings
• bun run build — succeeds, dist/ccstatusline.js produced
• 3 pre-existing keychain test failures confirmed present on main (not introduced by this PR)

Recent changes

• Security: Sanitize session ID before using in cache filename to prevent path traversal
• Reliability: Cache write failures are now best-effort — no longer crash the status line
• Tests: Added persistence round-trip, path traversal regression, and write failure tests

🤖 Generated with Claude Code