MVP: serviceize health, readiness, state, events, metrics, and repair planning endpoints

[mdheller]

Purpose

Define and implement the local-only service boundary for sourceos-syncd after persistence and model contracts exist.

Canonical architecture: SourceOS-Linux/sourceos-spec/docs/architecture/sourceos-state-integrity-layer.md
Program tracker: SourceOS-Linux/sourceos-spec#83

Intended local endpoints

• /healthz liveness
• /readyz readiness
• /statez structured State Integrity Report
• /events canonical event stream
• /events/{id} event evidence reference
• /events/{id}/explain operator narrative
• /metrics Prometheus/OpenTelemetry export
• /repairz repair planning endpoint; local-only and policy-gated

Requirements

• Local-only by default.
• No remote repair endpoint unless policy explicitly permits it.
• Readiness must distinguish healthy, degraded, unsafe, and uninitialized local state.
• /statez must be compatible with current State Integrity Report contract.
• Service layer must not replace CLI; CLI remains the stable operator path.

Acceptance criteria

• Service boundary is documented before implementation.
• Endpoints have JSON response examples.
• Readiness and liveness semantics are tested.
• Repair planning endpoint is preview-only until apply policy is designed.