feat: add embedded Cairnline read-model probe

[chicoxyzzy]

Summary

• add a local-only embedded Cairnline read-model probe that reads the existing mirror SQLite DB directly
• expose the probe URL in project coordination backend status and keep it blocked in remote runtime mode
• document the difference between snapshot-seeded read readiness and direct embedded mirror read readiness

Verification

• GOCACHE="/Users/chicoxyzzy/dev/hecate/hecate/.gocache" go test ./internal/api -run 'TestProjectCairnline(EmbeddedReadModel|MirrorParity|Sync|Export)|TestProjectCoordinationBackendStatus|TestRemoteRuntime'
• GOCACHE="/Users/chicoxyzzy/dev/hecate/hecate/.gocache" go vet ./internal/api
• just agent-docs-check
• git diff --check
• GOCACHE="/Users/chicoxyzzy/dev/hecate/hecate/.gocache" go test -race -timeout 10m ./...

Note: the first sandboxed race run was blocked by localhost listener permissions in an existing httptest server; the same command passed with normal local permissions.