refactor: write-skill phase 1 refactor — grill, implementation, lessons

Complete 2026-05-18 workstream for write-skill refactor (issue 0018 phase 1).

Grill output (handoff in docs/issues/0018-factory-write-skill.md):
- 6-section standard replaces 8: Role and When/When not dropped (not in
  agentskills.io spec; description carries trigger scope and negatives)
- Frontmatter reduced to 3 fields; provenance moves to META.md
  (progressive disclosure — not loaded at every skill scan)
- SKILL-TEMPLATE.md and META-TEMPLATE.md as authoritative copy-fill artifacts
- CATEGORIES.md as self-contained category reference in skill directory
- Copy and fill explicitly split into separate process steps for determinism

New files in .agents/skills/write-skill/:
- SKILL-TEMPLATE.md — 6-section template with XML blocks and inline examples per section
- META-TEMPLATE.md — provenance schema with inline-commented YAML; source vs references explicit
- CATEGORIES.md — self-contained category table (no runtime dependency on factory docs)
- META.md — write-skill's own provenance (v1.1, self-authored)

SKILL.md rewritten: 8-step process, XML blocks (<requirements>, <steps>, <checks>),
3-field frontmatter, copy-then-fill steps explicit.

Supporting docs updated:
- skill-implementation-workflow.md: SKILL-TEMPLATE.md as authoritative template;
  source:/references: now reference META.md; when: marked resolved
- factory-integration-decisions.md: frontmatter spec corrected
- ROADMAP.md: authoring standard description updated
- CONTEXT.md: Source field entry corrected; META.md added to glossary
- docs/spec/overview.md: 2026-05-18 refactor entry added
- docs/issues/0018: handoff status updated; acceptance criteria marked complete

LESSONS.md: 3 entries added in grill session (cargo-culted body sections,
provenance fields in frontmatter, copy-fill determinism) + 1 from implementation
(planning meta-commentary does not belong in deployed artifacts).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CONTEXT.md

@@ -93,7 +93,10 @@ Contains always-on rules in plain markdown with no provider-specific syntax (no
 A skill calling another skill by name to delegate a sub-task. The calling skill focuses on the orchestration decision ("when to do X"); the called skill owns the mechanics ("how to do X"). Established compositions: `grill-me` calls `write-adr` when a decision crystallises; `implement-feature` calls `tdd` as its implementation methodology. Composition chains are formalised as workflows in Chunk 4.
 
 ### Source field
-Frontmatter field (`source:`) on skill files tracking upstream provenance. An array — supports multiple upstream sources per skill. Each entry: `repo` (GitHub slug, e.g. `mattpocock/skills` — no URL, slug is stable and searchable), `commit` (exact SHA reviewed at adoption), `files` (list of files adopted with inline comments on what was taken), `updated` (date of last upstream review for this entry). Absence of `source:` means self-authored original. Upstream review cadence: per-skill during Chunk 3 (run during source review step); quarterly after roadmap completion (post Chunk 7). Companion field: `references:` (array of URLs or citations) for general external citations — distinct from `source:` which tracks adoptions with commit-level traceability.
+Field (`source:`) in a skill's `META.md` tracking upstream provenance. An array — supports multiple upstream sources per skill. Each entry: `repo` (GitHub slug, e.g. `mattpocock/skills` — no URL, slug is stable and searchable), `commit` (exact SHA reviewed at adoption), `files` (list of files adopted with inline comments on what was taken), `updated` (date of last upstream review for this entry). Absence of `source:` means self-authored original. Upstream review cadence: per-skill during Chunk 3 (run during source review step); quarterly after roadmap completion (post Chunk 7). Companion field: `references:` (array of URLs or citations) for general external citations — distinct from `source:` which tracks adoptions with commit-level traceability. Both fields live in `META.md`, not in SKILL.md frontmatter.
+
+### META.md
+A per-skill markdown file containing a single YAML code block with provenance and audit fields: `version`, `updated`, `when`, `source`, and `references`. Lives alongside the SKILL.md in the skill directory (`.agents/skills/<name>/META.md`). Not loaded at agent startup — progressive disclosure principle: name and description route the skill; provenance is only needed for upgrade reviews and audits. Prevents these fields from being scanned on every session start alongside every skill's name and description. The authoritative schema is `META-TEMPLATE.md` in `.agents/skills/write-skill/`. See also: [[Source field]].
 
 ### Provider-agnostic issue tracker
 Skills and workflows reference "linked issue" generically rather than a specific provider. In the file-based phase, an issue is a `docs/issues/NNNN-<slug>.md` file. When Gitea MCP is configured, the same skills use it instead. The active backend is determined at runtime by MCP availability. "Issue" is the canonical cross-provider term (GitHub, GitLab, Gitea all use it). Gitea-specific skills are a provider adapter (`providers/gitea/`), not part of the core library. See ADR-0011.