feat: add tokenless Tobserver change skill

tobserver-change/assets/pull-request-body.md

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+## Summary
+
+- <smallest declarative change and intended outcome>
+
+## Validation
+
+- `tobserver-agent check` — <exact result>
+- <additional focused check, if any>
+
+## Risks and limitations
+
+- <unverified behavior, platform limitation, or `None`>
+
+## Manual follow-up
+
+- Review and merge this pull request in Codeberg.
+- Deploy manually on Tobserver after merge.
+- <human-managed secret step, when applicable; otherwise remove>