refactor: move diagnostics into each skill's own directory

Replace shared _shared/diagnostics.md with dedicated diagnostics.md
files in setup/ and update-nanoclaw/. Each contains only the event
types relevant to that skill. References updated to local links.

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

.claude/skills/update-nanoclaw/SKILL.md

@@ -237,4 +237,4 @@ Tell the user:
 
 ## Diagnostics (Optional)
 
-After completing all steps above, read and follow `.claude/skills/_shared/diagnostics.md`.
+After completing all steps above, read and follow [diagnostics.md](diagnostics.md).

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

@@ -0,0 +1,59 @@
+# Diagnostics (end-of-update instructions)
+
+Follow these steps **after all other update steps are complete**.
+
+## 1. Build event data
+
+Estimate `error_count` from the conversation (how many errors/retries occurred).
+
+```json
+{
+  "version_age_days": 45,
+  "update_method": "merge",
+  "conflict_files": ["package.json"],
+  "breaking_changes_found": false,
+  "breaking_changes_skills_run": [],
+  "error_count": 0
+}
+```
+- `version_age_days`: estimate from the backup tag or commit date how many days old the previous version was
+- `update_method`: "merge" or "rebase"
+- `conflict_files`: filenames with merge conflicts (the script gates these against upstream)
+- `breaking_changes_found`: whether breaking changes were detected
+- `breaking_changes_skills_run`: which skills had to be re-run to fix breaking changes
+
+## 2. Dry run
+
+```bash
+npx tsx scripts/send-diagnostics.ts --event update_complete --success --data '<json>' --dry-run
+```
+
+Use `--failure` instead of `--success` if the update failed.
+
+If the dry-run produces no output, the user has opted out permanently — skip the rest.
+
+## 3. Show the user and ask
+
+> "Would you like to send anonymous diagnostics to help improve NanoClaw? Here's exactly what would be sent:"
+>
+> (show JSON payload)
+>
+> **Yes** / **No** / **Never ask again**
+
+Use AskUserQuestion.
+
+## 4. Handle response
+
+- **Yes**: Run without `--dry-run`:
+  ```bash
+  npx tsx scripts/send-diagnostics.ts --event update_complete --success --data '<json>'
+  ```
+  Confirm: "Diagnostics sent."
+
+- **No**: Do nothing. User will be asked again next time.
+
+- **Never ask again**: Run:
+  ```bash
+  npx tsx -e "import { setNeverAsk } from './scripts/send-diagnostics.ts'; setNeverAsk();"
+  ```
+  Confirm: "Got it — you won't be asked again."