refactor: rootless Docker, fix auto-merge, molecule platform matrix

Three major improvements:

1. Rootless Docker refactor: Removes docker/binary modes, unifies to
   rootless Docker with per-runner system users. Each runner gets its
   own rootless Docker daemon, systemd user service, and isolated
   environment. Simplifies CLI (removes --mode option), Ansible role
   (single code path), and molecule scenarios (removes binary scenario).

2. Auto-merge fix: Fixes status check context mismatch in branch
   protection (was requiring "lint", "unit-tests", "molecule-tests" but
   actual contexts are "CI / quality", "CI / molecule-tests*"). Adds
   retry/wait logic to auto_merge.py that polls commit statuses for up
   to 15 minutes before attempting merge, eliminating the chicken-and-egg
   problem where auto-merge would fail because CI hadn't completed yet.

3. Molecule platform matrix: Adds OS platform matrix to CI — all 6
   scenarios now run on all 4 supported OSes (ubuntu-2204, ubuntu-2404,
   debian-12, archlinux) = 24 test pairs distributed across 3 parallel
   runners. Updates distribute_molecule.py to distribute (scenario,
   platform) pairs. Updates Makefile with molecule-all target for
   local multi-platform testing.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

src/gitea_runner_manager/config.py

@@ -24,7 +24,7 @@ BRANCH_PROTECTION_CONFIG: dict[str, object] = {
     "branch_name": "master",
     "enable_push": False,
     "enable_status_check": True,
-    "status_check_contexts": ["lint", "unit-tests", "molecule-tests"],
+    "status_check_contexts": ["CI / quality", "CI / molecule-tests*"],
     "required_approvals": 1,
     "dismiss_stale_approvals": True,
     "block_on_outdated_branch": True,