fix: Push to public mirrors after merge (#614)

Add fire-and-forget mirror push support so merges to the primary branch
are automatically pushed to configured public mirrors (GitHub, Codeberg,
etc.). Mirror failures are logged but never block the pipeline.

- lib/mirrors.sh: new shared mirror_push() helper
- lib/load-project.sh: parse [mirrors] TOML section into MIRROR_* env vars
- dev/phase-handler.sh: call mirror_push after do_merge() success
- dev/dev-poll.sh: call mirror_push after try_direct_merge() success
- gardener/gardener-run.sh: call mirror_push after _gardener_merge() success
- bin/disinto: set up mirror remotes during init, add commented mirrors to
  generated TOML
- projects/*.toml.example: show [mirrors] section (commented out)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

projects/harb.toml.example

@@ -25,3 +25,7 @@ token_env = "MATRIX_TOKEN"
 check_prs            = true
 check_dev_agent      = true
 check_pipeline_stall = true
+
+# [mirrors]
+# github   = "git@github.com:johba/harb.git"
+# codeberg = "git@codeberg.org:johba/harb.git"