feat: Session Intelligence Layer — /checkpoint + /health + context recovery (v0.15.0.0) (#733)

* feat: session timeline binaries (gstack-timeline-log + gstack-timeline-read)

New binaries for the Session Intelligence Layer. gstack-timeline-log appends
JSONL events to ~/.gstack/projects/$SLUG/timeline.jsonl. gstack-timeline-read
reads, filters, and formats timeline data for /retro consumption.

Timeline is local-only project intelligence, never sent anywhere. Always-on
regardless of telemetry setting.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: preamble context recovery + timeline events + predictive suggestions

Layers 1-3 of the Session Intelligence Layer:
- Timeline start/complete events injected into every skill via preamble
- Context recovery (tier 2+): lists recent CEO plans, checkpoints, reviews
- Cross-session injection: LAST_SESSION and LATEST_CHECKPOINT for branch
- Predictive skill suggestion from recent timeline patterns
- Welcome back message synthesis
- Routing rules for /checkpoint and /health

Timeline writes are NOT gated by telemetry (local project intelligence).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: /checkpoint + /health skills (Layers 4-5)

/checkpoint: save/resume/list working state snapshots. Supports cross-branch
listing for Conductor workspace handoff. Session duration tracking.

/health: code quality scorekeeper. Wraps project tools (tsc, biome, knip,
shellcheck, tests), computes composite 0-10 score, tracks trends over time.
Auto-detects tools or reads from CLAUDE.md ## Health Stack.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* chore: regenerate SKILL.md files + add timeline tests

9 timeline tests (all passing) mirroring learnings.test.ts pattern.
All 34 SKILL.md files regenerated with new preamble (context recovery,
timeline events, routing rules for /checkpoint and /health).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* chore: bump version and changelog (v0.15.0.0)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: update self-learning roadmap post-Session Intelligence

R1-R3 marked shipped with actual versions. R4 becomes Adaptive Ceremony
(trust as separate policy engine, scope-aware, gradual degradation). R5
becomes /autoship (resumable state machine, not linear chain). R6-R7
unbundled from old R5. Added State Systems reference, Risk Register
(Codex-reviewed), and validation metrics for R4.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* test: E2E tests for Session Intelligence (timeline, recovery, checkpoint)

3 gate-tier E2E tests:
- timeline-event-flow: binary data flow round-trip (no LLM)
- context-recovery-artifacts: seeded artifacts appear in preamble
- checkpoint-save-resume: checkpoint file created with YAML frontmatter

Also fixes package.json version sync (0.14.6.0 → 0.15.0.0).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -1,5 +1,21 @@
 # Changelog
 
+## [0.15.0.0] - 2026-04-01 — Session Intelligence
+
+Your AI sessions now remember what happened. Plans, reviews, checkpoints, and health scores survive context compaction and compound across sessions. Every skill writes a timeline event, and the preamble reads recent artifacts on startup so the agent knows where you left off.
+
+### Added
+
+- **Session timeline.** Every skill auto-logs start/complete events to `timeline.jsonl`. Local-only, never sent anywhere, always on regardless of telemetry setting. /retro can now show "this week: 3 /review, 2 /ship across 3 branches."
+- **Context recovery.** After compaction or session start, the preamble lists your recent CEO plans, checkpoints, and reviews. The agent reads the most recent one to recover decisions and progress without asking you to repeat yourself.
+- **Cross-session injection.** On session start, the preamble prints your last skill run on this branch and your latest checkpoint. You see "Last session: /review (success)" before typing anything.
+- **Predictive skill suggestion.** If your last 3 sessions on a branch follow a pattern (review, ship, review), gstack suggests what you probably want next.
+- **Welcome back message.** Sessions synthesize a one-paragraph briefing: branch name, last skill, checkpoint status, health score.
+- **`/checkpoint` skill.** Save and resume working state snapshots. Captures git state, decisions made, remaining work. Supports cross-branch listing for Conductor workspace handoff between agents.
+- **`/health` skill.** Code quality scorekeeper. Wraps your project's tools (tsc, biome, knip, shellcheck, tests), computes a composite 0-10 score, tracks trends over time. When the score drops, it tells you exactly what changed and where to fix it.
+- **Timeline binaries.** `bin/gstack-timeline-log` and `bin/gstack-timeline-read` for append-only JSONL timeline storage.
+- **Routing rules.** /checkpoint and /health added to the skill routing injection.
+
 ## [0.14.6.0] - 2026-03-31 — Recursive Self-Improvement
 
 gstack now learns from its own mistakes. Every skill session captures operational failures (CLI errors, wrong approaches, project quirks) and surfaces them in future sessions. No setup needed, just works.