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>

bin/gstack-timeline-log

@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+# gstack-timeline-log — append a timeline event to the project timeline
+# Usage: gstack-timeline-log '{"skill":"review","event":"started","branch":"main"}'
+#
+# Session timeline: local-only, never sent anywhere.
+# Required fields: skill, event (started|completed).
+# Optional: branch, outcome, duration_s, session, ts.
+# Validation failure → skip silently (non-blocking).
+set -euo pipefail
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+eval "$("$SCRIPT_DIR/gstack-slug" 2>/dev/null)"
+GSTACK_HOME="${GSTACK_HOME:-$HOME/.gstack}"
+mkdir -p "$GSTACK_HOME/projects/$SLUG"
+
+INPUT="$1"
+
+# Validate: input must be parseable JSON with required fields
+if ! printf '%s' "$INPUT" | bun -e "
+  const j = JSON.parse(await Bun.stdin.text());
+  if (!j.skill || !j.event) process.exit(1);
+" 2>/dev/null; then
+  exit 0  # skip silently, non-blocking
+fi
+
+# Inject timestamp if not present
+if ! printf '%s' "$INPUT" | bun -e "const j=JSON.parse(await Bun.stdin.text()); if(!j.ts) process.exit(1)" 2>/dev/null; then
+  INPUT=$(printf '%s' "$INPUT" | bun -e "
+    const j = JSON.parse(await Bun.stdin.text());
+    j.ts = new Date().toISOString();
+    console.log(JSON.stringify(j));
+  " 2>/dev/null) || true
+fi
+
+echo "$INPUT" >> "$GSTACK_HOME/projects/$SLUG/timeline.jsonl"