docs(agents): add writing-docs guide and new-term procedure

docs/agents/writing-docs.md holds the doc/comment rules and done-check,
reached via a pointer in CLAUDE.md (AGENTS.md and QWEN.md import it).
docs/agents/domain.md gains the procedure for new or conflicting terms.

Refs #55 (agent guide #62)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/agents/domain.md

@@ -28,6 +28,14 @@ When your output names a domain concept (in an issue title, a refactor proposal,
 
 If the concept you need isn't in the glossary yet, that's a signal — either you're inventing language the project doesn't use (reconsider) or there's a real gap (note it for `/domain-modeling`).
 
+## New or conflicting terms
+
+When you meet a term that isn't in `CONTEXT.md`:
+
+- **A synonym of an existing term** (e.g. "proxy" for *gateway*): use the canonical term. There's nothing to edit or ask.
+- **A genuinely new concept:** add it to `CONTEXT.md` in the same change, as the term, a one- or two-sentence definition and an _Avoid_ list, with no values or config. Name the new term in your final message so the user can veto it.
+- **A conflict with an existing definition** (the code or the user uses a term differently than `CONTEXT.md` defines it): stop and ask the user. Redefining a term is their decision.
+
 ## Flag ADR conflicts
 
 If your output contradicts an existing ADR, surface it explicitly rather than silently overriding:

docs/agents/writing-docs.md

@@ -0,0 +1,23 @@
+# Writing docs
+
+How to write and update this repo's docs and code comments. The source is the truth: docs explain *why* and point at the source for *what*.
+
+## Rules
+
+1. **Source-of-truth line.** Every doc outside `docs/research/` and `docs/agents/` opens with a `> **Source of truth:** …` line. It names the files, services or keys the doc explains and says the source wins on disagreement. In the body, name keys and flags (`LLAMA_PARALLEL`, `--cache-reuse`) and leave their values in the source. When a doc and its source disagree, fix the doc in the same change.
+2. **The README index is the doc map.** Every new doc gets a row in `README.md`'s Docs table.
+3. **Code comments are pointers.** In `docker-compose.yml`, `.env.example` and `scripts/`, a comment is a one-line `# see docs/<file>.md#<section>` pointer, and the explanation goes in that doc section. Two things stay inline as one line, next to the pointer:
+   - a **warning** about data loss, security, or an edit that breaks the stack or a script
+   - on `.env.example` keys that `update.sh` fills in, the marker "auto-filled by update.sh — leave blank"
+
+   A `ponytail:` note becomes a pointer too, with its reasoning moved into the doc.
+4. **A source change carries its doc.** When you change code that has a pointer, update the doc section it names in the same change.
+5. **`docs/research/` is frozen history.** New research goes in a new file plus a row in [`docs/research/README.md`](../research/README.md). When a research doc goes stale, change its status in that index and leave the file itself untouched.
+
+Terminology follows [`CONTEXT.md`](../../CONTEXT.md). For a term that isn't there, see [domain docs → New or conflicting terms](domain.md#new-or-conflicting-terms).
+
+## Done when
+
+- every pointer in the code you touched resolves to an existing heading
+- every doc you touched opens with its source-of-truth line
+- no doc you touched restates a value from its source