refactor(services): return the incident ID from CreateIncident

[nghiadaulau]

Summary

services.CreateIncident generated and persisted an incident ID but threw
it away, returning only error. This changes the signature to
(incidentID string, err error) and returns the ID, so callers can act on
the incident they just created.

Why

The service layer is the only place that holds the new incident's ID (the
worker's Emitter func type does not). Returning it here is the enabling
seam for follow-up work such as auto-analysis on emission, and is generally
useful (e.g. logging/linking the created incident).

The ID is returned even when a downstream channel or on-call step fails,
because the incident is persisted before fan-out and therefore exists; an
empty ID is returned only when no incident was created (provider
construction failed before persistence).

Changes

• pkg/services/incident.go — signature + returns.
• Call sites updated to the new signature (most ignore the ID with _,):
  cmd/main.go, pkg/controllers/incident.go (×4),
  pkg/controllers/sns.go (×2), pkg/services/agent.go.
• No behavior change beyond the return value; no config/schema change.

Testing

• TestCreateIncident_ReturnsPersistedID — loads a minimal config + memory
  store, creates an incident with all channels disabled, and asserts the
  returned ID is non-empty and resolves to the persisted record.
• gofmt, go vet, go build ./..., go test -race ./pkg/services ./pkg/controllers
  green.

Checklist

• Test for the new return value
• All call sites updated; compiles
• No config change; no user-facing behavior change
• House conventions (refactor: prefix); CHANGELOG updated