feat(skills): add skill entity seed schema and boot-time registration

[castor-agent]

Summary

• Adds src/services/skills/seed_schema.ts — idempotent boot-time schema seeder for the skill entity type, following the same pattern as plans/seed_schema.ts and issues/seed_schema.ts
• Wires seedSkillSchema() into actions.ts alongside issue and plan seeds so the global skill schema is guaranteed on every server start
• Covers all 11 skill fields: name, description, triggers, content, slug, user_invocable, enabled, version, supported_harnesses, harness_config, synced_at
• canonical_name_fields: ["name"] — one row per skill name per instance
• user_specific: false (global schema), migrate_existing: true for incremental field backfills
• Adds docs/developer/skills_guide.md — user-facing guide for skills (storage, retrieval, mirror, slash-palette), in line with the feature guides added in docs: add user-facing feature guides (plans, issues, mirror, subs) #314
• Adds Feature guides section to docs/developer/getting_started.md with skills alongside plans, issues, mirror, and subscriptions

Relationship to #314

PR #314 adds user-facing feature guides for plans, issues, mirror, and subscriptions. This PR adds the equivalent guide for skills and includes the same Feature guides section in getting_started.md. These two PRs will have a minor conflict on getting_started.md (both add the same section header) — whichever merges second should keep both entries.

Test plan

• Fresh server start seeds skill schema without error
• Re-running seed on existing schema is a no-op (idempotent)
• Adding a new field to SKILL_FIELDS on a running instance triggers incremental update and migrates existing rows
• docs/developer/skills_guide.md renders on /docs route

🤖 Generated with Claude Code

[castor-agent]

/create_pr skill audit

Automated audit against the /create_pr skill (Neotoma plan ent_8cde942847555a83253dfe5b), which defines the standard PR description structure (Problems / Solutions / UX improvements / Documentation / Test plan / Breaking changes), the pre-PR checklist from .claude/rules/change_guardrails_rules.md, and the functional-change documentation gate (docs entry + docs/site/site_doc_manifest.yaml + docs server surfacing).

Verdict: does not fully meet standards. Gaps below; recommended changes follow each item.

Findings

• Missing required sections: Problems, Solutions, UX improvements, Documentation, Breaking changes.
• Breaking changes section MUST be present, even for non-breaking PRs (write No breaking changes.).
• docs/site/site_doc_manifest.yaml is not updated. New or substantively changed docs must be registered there so the docs site surfaces them.
• PR body does not confirm the doc is reachable via the docs server (src/services/docs/) or list the specific docs/...md path under the Documentation section.
• No Related section and no Fixes #N / Closes #N reference — link the plan and/or issue(s) so the PR closes them on merge.

Recommended changes

Update the PR description to match the skill template exactly:

## Problems
- <Concrete pain point or gap.>

## Solutions
- <Concrete change made.>

## UX improvements
- <User-visible behavior change, or `No user-visible change.`>

## Documentation
- <docs/... path(s) added or updated; parameters/outputs/examples/error modes covered.>
- <`docs/site/site_doc_manifest.yaml` entry added/updated; docs service tests pass.>
- <Or: `No functional change; no user-facing docs required.`>

## Test plan
- [ ] `npm run type-check`
- [ ] `npm test`
- [ ] `npm test -- src/services/docs` (if docs changed)
- [ ] Manual verification: <steps>

## Breaking changes
No breaking changes.

## Related
- Plan: <Neotoma plan entity_id or docs/ path>
- Issue(s): <#N>

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Functional surfaces detected from the diff: MCP. Per the skill, the Documentation section must point at a real docs/...md path covering parameters, outputs, at least one example, and error modes, and the doc must be listed in docs/site/site_doc_manifest.yaml so it is surfaced by the docs server and docs site.

Posted by /create_pr skill audit run. See .claude/skills/create_pr/SKILL.md for the full template.