feat(onboarding): surface telemetry opt-out path during /onboard-observability and in skill docs

## Summary

`/vsdd-factory:onboard-observability` enables 5 OTEL env vars in `.claude/settings.local.json`, sending Claude Code's own telemetry (tool calls, costs, tokens) to the local OTel collector. The skill never mentions:

- How to disable telemetry later
- That a `/vsdd-factory:claude-telemetry off` skill exists for this exact purpose
- That `.claude/settings.local.json` is the file that holds the toggle
- That data already shipped to local Loki / Prom stays until its retention expires

## Repro

Run `/vsdd-factory:onboard-observability`. The final summary tells the operator to restart their session and `factory-obs up`. Nothing about opt-out.

## Proposed

Add to the skill's "Next steps" output:

```
Telemetry control:
- To disable later: /vsdd-factory:claude-telemetry off
- Config location: .claude/settings.local.json (5 OTEL_* keys)
- Already-shipped data: lives in local Loki/Prom; retention is configured
in the docker compose stack (default: 30 days). To clear immediately:
factory-obs reset --include-data
```

And in `skills/onboard-observability/SKILL.md`, add a "Reversibility" section.

## Acceptance criteria

- [ ] `/onboard-observability` output explicitly references the opt-out skill.
- [ ] SKILL.md has a "Reversibility" or "How to disable later" section.
- [ ] The opt-out skill's docs cross-reference the onboard skill.

## Found during

`/vsdd-factory:onboard-observability` on `switchboard-blue` (2026-06-23, vsdd-factory@1.0.0-rc.21). No mention of how to undo. I had to search the skill list to find `/vsdd-factory:claude-telemetry off`.