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.

AGENTS.md

@@ -1,6 +1,6 @@
 ## Project Overview
 
-**Unitdore** is a Go CLI tool that bridges container runtimes (Podman, Docker) and systemd. It discovers running containers, stores them in a YAML config, and generates + manages systemd `.service` unit files for each one.
+**Unitdore** is a Go CLI tool that bridges container runtimes (Podman, Docker) and the init system (systemd, OpenRC, runit). It discovers running containers, stores them in a YAML config, and generates + manages service files for each one.
 
 # Agent Rules
 
@@ -33,7 +33,11 @@ config/        # Config struct + YAML load/save (units.yaml)
 runtime/       # Docker and Podman runtime abstraction, compose labels
 sysuser/       # run commands as root/user (runuser), home/linger lookup
 testdata/stack # podman-compose test stack + captured `podman ps` fixture
-systemd/       # .service file generator and systemd manager
+service/       # init-neutral unit resolution: commands, quoting, user env
+initsys/       # Init interface, backend selection (config init: / autodetect)
+systemd/       # .service generator + systemctl manager (Backend)
+openrc/        # /etc/init.d script generator + rc-service manager (Backend)
+runit/         # /etc/sv run/finish generator + sv manager (Backend)
 docs/          # generated-units.md (unit file examples), unitdore.1 (man page)
 ```
 
@@ -41,13 +45,18 @@ docs/          # generated-units.md (unit file examples), unitdore.1 (man page)
 
 - **`config`** — `Unit` and `Config` structs; `Load`, `Save`, `FindUnit`, `AddUnit`
 - **`runtime`** — `ContainerRuntime` interface; `docker.go`, `podman.go` implementations
+- **`service`** — `Resolve(unit)` → start/stop commands, type, workdir; shared by all backends
+- **`initsys`** — `Init` interface; `New(cfg)` picks backend; cmd/ only talks to `Init`
 - **`systemd`** — `generator.go` builds `.service` file content; `manager.go` calls `systemctl`
+- **`openrc`**, **`runit`** — same role; root only; tests stub command execution via package vars
 
 ### Config file
 
 **Location:** `/etc/unitdore/units.yaml`
 
 ```yaml
+init: ""        # systemd | openrc | runit (empty = autodetect)
+service_dir: ""  # runit: supervised dir (empty = autodetect)
 prefix: ""      # prepended to service file name (e.g. "prod-")
 suffix: ""      # appended to service file name (e.g. "-svc")
 users: []       # root only: rootless users scanned by syncup
@@ -101,6 +110,10 @@ Tests exist in:
 - `runtime/runtime_test.go`
 - `cmd/syncup_test.go`
 - `sysuser/sysuser_test.go`
+- `service/spec_test.go`
+- `initsys/initsys_test.go`
+- `openrc/openrc_test.go`
+- `runit/runit_test.go`
 - `systemd/generator_test.go`
 
 Always run `make test` before marking a task complete. Add tests for new behaviour in the same package as the code under test.