feat: add diagnostics telemetry to migrate-nanoclaw skill

Matches the pattern used by /setup and /update-nanoclaw. Captures
migration-specific properties (tier, phase, customization count,
skill interactions). Opt-out permanently disables across all skills.

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

.claude/skills/migrate-nanoclaw/diagnostics.md

@@ -0,0 +1,52 @@
+# Diagnostics
+
+Gather system info:
+
+```bash
+node -p "require('./package.json').version"
+uname -s
+uname -m
+node -p "process.versions.node.split('.')[0]"
+```
+
+Write `/tmp/nanoclaw-diagnostics.json`. No paths, usernames, hostnames, or IP addresses.
+
+```json
+{
+  "api_key": "phc_fx1Hhx9ucz8GuaJC8LVZWO8u03yXZZJJ6ObS4yplnaP",
+  "event": "migrate_complete",
+  "distinct_id": "<uuid>",
+  "properties": {
+    "success": true,
+    "nanoclaw_version": "1.2.43",
+    "os_platform": "darwin",
+    "arch": "arm64",
+    "node_major_version": 22,
+    "migration_phase": "extract|upgrade|both",
+    "tier": 2,
+    "customization_count": 3,
+    "skills_applied_count": 2,
+    "skill_interaction_count": 0,
+    "live_test": false,
+    "breaking_changes_found": false,
+    "error_count": 0
+  }
+}
+```
+
+Show the entire JSON to the user and ask via AskUserQuestion: **Yes** / **No** / **Never ask again**
+
+**Yes**:
+```bash
+curl -s -X POST https://us.i.posthog.com/capture/ -H 'Content-Type: application/json' -d @/tmp/nanoclaw-diagnostics.json
+rm /tmp/nanoclaw-diagnostics.json
+```
+
+**No**: `rm /tmp/nanoclaw-diagnostics.json`
+
+**Never ask again**:
+1. Replace contents of `.claude/skills/setup/diagnostics.md` with `# Diagnostics — opted out`
+2. Replace contents of `.claude/skills/update-nanoclaw/diagnostics.md` with `# Diagnostics — opted out`
+3. Replace contents of `.claude/skills/migrate-nanoclaw/diagnostics.md` with `# Diagnostics — opted out`
+4. Remove the diagnostics sections from each corresponding SKILL.md
+5. `rm /tmp/nanoclaw-diagnostics.json`