Add migration plan for Forgejo brew-to-source transition (#140)

## Summary
- Add `docs/how-to/plans/migrate-forgejo-from-brew.md` — full Diataxis-style plan covering background, one-time migration steps, Ansible role changes (with exact code), verification checklist, and future considerations
- Add `docs/how-to/plans/plans.md` — new plans subdirectory index for upcoming migration/transition plans
- Update `docs/how-to/how-to.md` with a Plans section
- Update `docs/tutorials/exploring-the-docs.md` to mention plans in the doc structure table and quick-path sections for Owner and AI audiences

## Test plan
- [x] `docs-check-links` passes
- [x] `docs-check-index` passes
- [x] All pre-commit hooks pass

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/140

docs/how-to/how-to.md

@@ -43,3 +43,12 @@ Task-oriented instructions for common BlumeOps operations. These guides assume y
 | [[restart-indri]] | Safely shut down and restart indri |
 | [[manage-flyio-proxy]] | Deploy, shutoff, and troubleshoot the public proxy |
 | [[troubleshooting]] | Diagnose and fix common issues |
+
+## Plans
+
+Migration and transition plans for upcoming infrastructure changes.
+
+| Plan | Description |
+|------|-------------|
+| [[plans]] | Index of all plans |
+| [[migrate-forgejo-from-brew]] | Transition Forgejo from Homebrew to source-built binary |