feat(service): add an opt-in supervisor backend for systemd-less hosts (#479)

[parkjs101]

What

jaw service --backend supervisor generates a keep-alive loop for hosts with no service manager (container with PID 1 = tini, no systemd). This is the layer that absorbs the ensure-jaw.sh + 60s while-loop the #479 reporter wrote by hand.

Two load-bearing details, both found by running the script rather than reading it

1. Liveness must come from jaw service status, not pgrep.
service status verifies the pidfile against the OS process start time, so a recycled PID reads as dead. A name-matching check cannot see PID recycling and would sit forever beside a dead server — the same false-healthy reading #479 is about. The supervisor branch's status therefore exits 1 when the server is down; the systemd branch's status exits 0 either way, which a loop must not trust.

2. setsid is util-linux — absent on macOS and minimal images.
Running the loop showed every start failing with setsid: command not found, i.e. the supervisor started nothing at all. It is now probed once, with nohup alone as the fallback.

Behavioral verification

Ran the generated loop against a recorder stub:

Scenario	Observed
server alive	3 status queries, 0 starts
server dead	a start every cycle, 0 errors logged
dead → recovers	starts stop once liveness returns
SIGTERM	exits within one interval, logs why

Plus sh -n syntax validation (POSIX sh, not bash — minimal images ship dash).

Scope note

detectBackend() does not select this automatically, and a test enforces that. Writing a script and telling the operator where to wire it in is a different act from registering autostart; implying otherwise would repeat the failure #479 reported.

11 unit tests (tests/unit/supervisor-service.test.ts), tsc --noEmit clean.

Refs #479

Stack (merge bottom-up):

#	PR	Layer	Review focus
4	#488	docs — remote/headless guide	docs only
3	#487	supervisor backend ← you are here	generated sh + liveness contract
2	#486	service guidance	operator message correctness
1	#485	doctor detection	detection logic + probe seam

Review this PR's diff only; its base is the layer below.

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