feat: add skill usage telemetry to preamble

TemplateContext system passes skill name through resolver pipeline so
each generated SKILL.md gets its own name baked into the telemetry line.
Appends to ~/.gstack/analytics/skill-usage.jsonl on every invocation.

Covers 14 preamble-using skills + 4 hook skills (inline telemetry).
JSONL format: {"skill":"ship","ts":"...","repo":"my-project"}

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

retro/SKILL.md

@@ -30,6 +30,8 @@ _BRANCH=$(git branch --show-current 2>/dev/null || echo "unknown")
 echo "BRANCH: $_BRANCH"
 _LAKE_SEEN=$([ -f ~/.gstack/.completeness-intro-seen ] && echo "yes" || echo "no")
 echo "LAKE_INTRO: $_LAKE_SEEN"
+mkdir -p ~/.gstack/analytics
+echo '{"skill":"retro","ts":"'$(date -u +%Y-%m-%dT%H:%M:%SZ)'","repo":"'$(basename "$(git rev-parse --show-toplevel 2>/dev/null)" 2>/dev/null || echo "unknown")'"}'  >> ~/.gstack/analytics/skill-usage.jsonl 2>/dev/null || true
 ```
 
 If output shows `UPGRADE_AVAILABLE <old> <new>`: read `~/.claude/skills/gstack/gstack-upgrade/SKILL.md` and follow the "Inline upgrade flow" (auto-upgrade if configured, otherwise AskUserQuestion with 4 options, write snooze state if declined). If `JUST_UPGRADED <from> <to>`: tell user "Running gstack v{to} (just updated!)" and continue.
@@ -263,6 +265,14 @@ Include in the metrics table:
 
 If TODOS.md doesn't exist, skip the Backlog Health row.
 
+**Skill Usage (if analytics exist):** Read `~/.gstack/analytics/skill-usage.jsonl` if it exists. Filter entries within the retro time window by `ts` field. Separate skill activations (no `event` field) from hook fires (`event: "hook_fire"`). Aggregate by skill name. Present as:
+
+```
+| Skill Usage | /ship(12) /qa(8) /review(5) · 3 safety hook fires |
+```
+
+If the JSONL file doesn't exist or has no entries in the window, skip the Skill Usage row.
+
 ### Step 3: Commit Time Distribution
 
 Show hourly histogram in Pacific time using bar chart: