Add observability-and-monitoring skill

[googlarz]

Summary

Adds observability-and-monitoring — the one confirmed lifecycle gap in the collection (no existing PR, not covered by any current skill). Covers the full production visibility workflow: structured logging, four golden signals metrics, OpenTelemetry distributed tracing, SLO-based alerting, and dashboard design.

Changes

• New skill: skills/observability-and-monitoring/SKILL.md
• New reference: references/observability-checklist.md — logging, metrics, tracing, SLOs/alerting, dashboards, pre-launch gate
• incremental-implementation: one-liner anchoring observability to slice definition ("define what you'll measure when you define the slice")
• using-agent-skills: decision tree, lifecycle (step 11, before shipping), quick reference table
• idea-refine: forward reference to spec-driven-development
• deprecation-and-migration: references to incremental-implementation, test-driven-development, git-workflow-and-versioning
• README: skill count 20 → 21, new entry in Ship section and directory tree

Review feedback addressed (from #59)

Feedback	Fix
Decision tree trigger is reactive	Reworded to "Instrumenting a service / setting up monitoring?"
Observability only appears at step 11	Added anchor in incremental-implementation — define measurements at slice definition time
No references/observability-checklist.md	Created — consistent with security-checklist.md, testing-patterns.md, performance-checklist.md
Steps assume greenfield	Added Step 0: brownfield audit — inventory existing instrumentation before adding more

Reviews run before submission

This branch went through /codex:adversarial-review and /codex:review before the original PR #59. Both findings were fixed (metric cardinality labelling, OTel propagation, /ship command wiring, lifecycle ordering). This PR is the observability-only split of that work.

[mvn-dongho-dn]

Implementation complete (parallel build).

• Commit: 90ae4dc
• Files changed: 8 (shared schema, Mongoose model, service, routes, 2 test files + 2 index updates)
• Tests: 372 passing (shared: 203, api: 144, web: 25)
• Merged with: Task 24 (parallel group: api-late)

Key points:

• Version numbers auto-increment per document starting at 1
• Diff absent on v1, computed via line-diff from v2 onward (no extra deps)
• Restore creates a NEW version (history is immutable)
• Editors can snapshot/restore; viewers get 403

→ Ready for dev review.

[googlarz]

Rebased on main after v0.6.0 merge. Key conflict resolved: /ship was completely rewritten as a fan-out orchestrator — integrated the observability step into Phase B (step 6) rather than the old numbered checklist. README skill count updated to 22.