fix(context): reset context length after a /compact boundary

[wmaurer]

Problem

Context-length / context-percentage widgets stay stuck at the pre-compaction token count after a /compact, until the next assistant turn — the bug reported in #92.

Right after /compact, Claude Code sets context_window.current_usage to null, so the context widgets fall back to transcript parsing. getTokenMetrics() then derives context length from the most recent usage entry — the now-defunct pre-compaction turn — so the displayed size never drops.

When #92 was closed, the reason given was that /compact "doesn't send anything back that gets logged to the jsonl." That's no longer true: Claude Code writes a { type: 'system', subtype: 'compact_boundary', compactMetadata: { preTokens, postTokens } } record on every compaction — the same marker the Compaction Counter widget already parses.

Fix

getTokenMetrics() is now compaction-aware. Usage entries before the most recent compact_boundary describe a context that no longer exists, so they no longer drive context length. Resolution order for contextLength:

1. the first main-chain turn after the boundary (once another message is sent), else
2. the boundary's compactMetadata.postTokens, else
3. 0 (older transcripts without postTokens) — never the stale pre-compaction number.

Session token totals (inputTokens, outputTokens, cachedTokens, totalTokens) stay cumulative — only contextLength resets. Behavior with no compaction is unchanged.

Boundary detection is reused from compaction.ts (isCompactBoundary is now exported, plus a small getCompactBoundaryPostTokens helper), so there's no duplicated parsing and no second pass over the transcript.

Tests

Added 3 cases to jsonl-metrics.test.ts:

• post-compaction size taken from postTokens before the next turn,
• first post-boundary turn supersedes the postTokens estimate,
• marker without postTokens + no following turn → 0 (no stale leak).

bun test (full suite) and bun run lint (tsc + eslint, --max-warnings=0) pass.

Before / after

Post-/compact, boundary postTokens = 18000, pre-compaction context ~235k:

	status line
before	... 235.0k (23.5%)
after	... 18.0k (1.8%)