feat: OpenRC and runit init backends behind an Init interface

Extract init-neutral unit resolution into service/, add initsys.Init with systemd, openrc and runit backends, selected by 'init:' config or autodetect. OpenRC and runit are root only; rootless units run as system services as that user. Add docs and generated examples for all three.

README.md

@@ -2,7 +2,7 @@
 
 > *A door you open and close for container units.*
 
-Unitdore bridges your container runtime (Podman, Docker) and systemd. It discovers running containers and podman-compose stacks, stores them in a config file, and generates + manages systemd `.service` units for each one.
+Unitdore bridges your container runtime (Podman, Docker) and your init system (systemd, OpenRC, runit). It discovers running containers and podman-compose stacks, stores them in a config file, and generates + manages a service for each one.
 
 ## Install
 
@@ -107,6 +107,8 @@ oldthing   podman    hein    no        no          —                 container
 **Location:** `/etc/unitdore/units.yaml`
 
 ```yaml
+init: ""                     # optional: systemd | openrc | runit (empty = autodetect)
+service_dir: ""              # optional: runit supervised dir (empty = /var/service or /etc/service)
 prefix: ""                   # optional: prepended to all service names, e.g. "prod-"
 suffix: ""                   # optional: appended to all service names, e.g. "-svc"
 users: [hein]                # root only: rootless users scanned by syncup
@@ -158,6 +160,27 @@ units:
     enabled: true
 ```
 
+### Init systems
+
+Set `init:` in the config, or leave empty to autodetect (`/run/systemd/system` → systemd, `/run/openrc` → openrc, `/etc/runit` → runit, else systemd).
+
+| | systemd | openrc | runit |
+|---|---|---|---|
+| Service file | `/etc/systemd/system/unitdore-<n>.service` | `/etc/init.d/unitdore-<n>` | `/etc/sv/unitdore-<n>/{run,finish}` + symlink in `service_dir` |
+| Start / stop | `systemctl` | `rc-service` | `sv` |
+| Enable at boot | `systemctl enable` | `rc-update add … default` | remove `down` file |
+| Installed state | disabled until enabled | not in runlevel | symlinked with `down` file (no autostart) |
+| Rootless `user:` | `systemctl --user` unit (needs linger) | system service, `command_user` | system service, `chpst -u` |
+| Non-root use | yes (own user units) | **root only** | **root only** |
+| `restart*` | `Restart=on-failure`, `StartLimitBurst` | `supervise-daemon` `respawn_*` (always respawns) | `finish`: on-failure, `RestartSec`; **no retry limit** |
+| `docker` dependency | `Requires=docker.service` | `need docker` | none |
+| `oneshot` | `RemainAfterExit` | `start()`/`stop()` functions | run once, then `tail -f /dev/null` |
+
+- Rootless units on openrc/runit set `HOME` and `XDG_RUNTIME_DIR=/run/user/<uid>` and create that dir if missing.
+- `command` / `args` use systemd quoting; they are re-quoted for sh on openrc/runit.
+- No log redirection is generated on openrc; use the runtime's own logs.
+- `docs/generated-units.md` shows systemd output only.
+
 ### Script / executable units
 
 Added manually (`unitdore edit`); syncup never creates or disables them.