feat: datetime reports, auto-commit on verify, register pruning EP

- Include time in TAP report filename (ISO 8601: date + HHmmssZ)
- Add changed_when: false to report write task — verify play now shows
  changed=0 on a clean run (all green recap)
- make verify auto-commits new reports to repo after a passing run;
  exits non-zero before committing if assertions fail
- Register EP-RAIL-001: report pruning extension point for future
  implementation when reports/ accumulates beyond a threshold

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

contrib/extension-points/EP-RAIL-001--railiance-hosts--goss-report-pruning.md

@@ -0,0 +1,53 @@
+---
+type: extension-point
+id: EP-RAIL-001
+title: "Goss TAP report pruning"
+target_org: railiance
+target_repo: railiance-hosts
+status: open
+created: "2026-03-09"
+source_repo: railiance-hosts
+related_workstream_id: ""
+---
+
+# EP-RAIL-001: Goss TAP Report Pruning
+
+## Context
+
+`make verify` commits a new TAP report file to `reports/` on every run:
+
+```
+reports/goss-Railiance01-2026-03-09T154855Z.tap
+```
+
+As the fleet grows and verify runs more frequently, `reports/` will accumulate
+indefinitely and bloat the repository history.
+
+## Extension Point
+
+Add a `make prune-reports` target (or integrate into `make verify`) that:
+
+- Keeps the N most recent reports per host (suggested default: N=30)
+- Removes older files and commits the deletion
+- Is configurable via a Makefile variable (`REPORTS_KEEP ?= 30`)
+
+Suggested implementation sketch:
+
+```makefile
+REPORTS_KEEP ?= 30
+
+prune-reports: ## Remove old Goss TAP reports, keep REPORTS_KEEP most recent per host
+	@for host in $$(ls reports/goss-*.tap 2>/dev/null | sed 's|reports/goss-||;s|-[0-9T]*Z\.tap||' | sort -u); do \
+	  ls -t reports/goss-$$host-*.tap | tail -n +$$(($(REPORTS_KEEP)+1)) | xargs -r rm -v; \
+	done
+	@git add reports/ && git diff --cached --quiet || \
+	  git commit -m "chore: prune Goss reports older than $(REPORTS_KEEP) per host"
+```
+
+## Trigger
+
+Implement when any of the following is true:
+
+- `reports/` contains more than 200 files, OR
+- Repository size grows noticeably due to reports, OR
+- verify is integrated into CI with high frequency runs