docs: trim CONTEXT.md to a glossary and demote VISION.md

Why: CONTEXT.md is an instructed every-session read at 33,799 bytes (~8,450
tokens), and CONTEXT.md:17 layered docs/VISION.md on top. After the AGENTS.md
trim, that left ~9,650 tokens of instructed reads sitting behind a file whose
own format contract — grill-with-docs/CONTEXT-FORMAT.md:43, "Keep definitions
tight. One sentence max." — it was not following. 22 glossary entries averaged
190 words; the top four were 55% of the file, and one ("Vale audit prefilter")
was 1,444 words of pre-commit forensics filed as a definition.

Implementation notes:
- CONTEXT.md rewritten to CONTEXT-FORMAT.md's prescribed template: ## Language
  (five clusters, 22 terms with _Avoid_ aliases), ## Relationships,
  ## Example dialogue, ## Flagged ambiguities. 33,799 -> 9,762 bytes.
- Vale prefilter forensics moved to docs/spec/gates.md under nine subsections:
  the two-copy style layout, vale-wrap.sh and the Vale 3.15.2
  text.frontmatter.description limitation, the --config argv defect (E100),
  glob scoping, the rule inventory, and 0-files-means-NOT-RUN.
- Instruction-file format and the bidirectional-reference principle moved to
  docs/spec/architecture.md, the latter flagged as aspirational: no instruction
  file carries a `when:` field, only 2 of 39 skills do, and the reference
  scanner script it names does not exist.
- Added a per-plugin scope table to architecture.md so "which plugin owns this
  skill" survives the trim, and corrected its claim that the content index
  lives in providers/claude-code/CLAUDE.md — that file is now two import lines
  and the index sits in core/AGENTS.md.
- "Repo and Gitea are the only source of truth" moved from CONTEXT.md's
  Principles into AGENTS.md session rules; it was operative guidance in a
  glossary. docs/VISION.md and LESSONS.md added to the on-demand list.
- The apm-consumed install entry (432 words) was deleted outright: all four
  consequences are already carried by README.md, ADR-0018, gates.md and
  AGENTS.md.
- ADR-0013, ADR-0015 and ADR-0017 each pointed at CONTEXT.md sections that
  moved; amended with dated supersession notes rather than left dangling.

Impact: the every-session set drops from ~15,700 to ~4,140 tokens across both
passes (-74%). The deleted "lint plugin" entry was also stale on a safety
claim — it said lint-runner inherits Edit, but ADR-0016 restored
`disallowedTools: Edit, Write, NotebookEdit` and CONTEXT.md was never updated.

Refs: #99

AGENTS.md

@@ -41,14 +41,17 @@ Fall back to raw shell only when no skill covers it.
 - **Before pushing, run the whole gate locally:** `pre-commit run --hook-stage pre-push --all-files`. Pushing runs 14 repo-defined hooks, not just the test suite.
 - **Pushing without a network** needs `SKIP=apm-marketplace-check,apm-pack-check-clean git push` — those two resolve a remote marketplace entry via `git ls-remote`. Skip only those two; the rest are real local checks, and adding one to `SKIP` disarms it silently.
 - **Author commits with `git-commits`** — it validates Conventional Commits, which `commit-msg` enforces.
+- **This repo and Gitea are the only source of truth.** All project state, decisions, and working conventions live here. Do not use an external memory system for this project — cached state diverges from the repo and you get a split brain. Before answering any design or architecture question, check `docs/adr/` for an existing decision.
 
 ## Key documents
 
-Read `CONTEXT.md` at the start of every session.
+Read `CONTEXT.md` at the start of every session — it is this repo's domain language, and the terms in it are used unglossed everywhere else.
 
 Read these on demand:
 
 - `README.md` — prerequisites, install, and test commands
+- `docs/VISION.md` — the phased roadmap and where this is going; read when a decision turns on product direction
+- `LESSONS.md` — patterns that went wrong once; read before repeating a class of change that has burned the repo before
 - `docs/spec/gates.md` — what each pre-commit and pre-push hook enforces and why; read when a gate fails or before changing hook config
 - `docs/spec/architecture.md` — directory structure, install pipeline, provider model
 - `docs/adr/` — architectural decisions; read before answering design questions or proposing structural changes