Add plans for Dagger CI/CD and upstream fork strategy (#150)

## Summary

Two new plan documents in `docs/how-to/plans/`:

- **adopt-dagger-ci** — Migrate CI/CD build logic from Forgejo Actions YAML to Dagger (Python SDK). Forgejo Actions stays as a thin trigger layer. Covers:
  - Container builds with local iteration (`dagger call build ... terminal`)
  - Docs builds with Forgejo packages migration (replacing Forgejo releases)
  - Runner simplification (only Docker + dagger CLI needed)
  - Secrets handling via Dagger's `Secret` type
  - Future: forked project builds, Python packages, pre-merge validation

- **upstream-fork-strategy** — Stacked-branch pattern for maintaining forks of upstream projects. Covers:
  - Daily automated rebase with conflict detection and issue creation
  - Branch model: `upstream/main` → `blumeops` → `feature/*`
  - Quartz fork as first instance, enabling `last-reviewed` frontmatter rendering in docs
  - Upstream PR path for contributing changes back

## Context

These plans emerged from evaluating alternatives to the GHA ecosystem (BuildKite, Concourse, Earthly) for CI/CD. Dagger was chosen for its local iteration story, Python-native pipelines, and zero-infrastructure requirements. The fork strategy is a prerequisite for customizing Quartz and other upstream tools.

Neither plan is ready for execution yet — they are design documents for future work.

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/150

docs/how-to/how-to.md

@@ -54,3 +54,5 @@ Migration and transition plans for upcoming infrastructure changes.
 | [[plans]] | Index of all plans |
 | [[migrate-forgejo-from-brew]] | Transition Forgejo from Homebrew to source-built binary |
 | [[add-unifi-pulumi-stack]] | Add Pulumi IaC for UniFi Express 7 |
+| [[adopt-dagger-ci]] | Adopt Dagger as CI/CD build engine |
+| [[upstream-fork-strategy]] | Stacked-branch forking strategy for upstream projects |