Add docs conventions to prevent near-duplicate markdown.

Document one-concept-one-file rules, canonical naming, amend-not-sibling, explicit deletes, and path announcement. No CI/hook infrastructure present yet.

docs/CONVENTIONS.md

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+# Docs conventions (anti-duplication)
+
+**Branch applicability:** whole repo; written on `planning-selfserve-audit` after repeated near-duplicate markdown sprawl.  
+**Scope:** `docs/**/*.md` (and any future top-level strategy markdown).
+
+These rules exist so agents and humans **do not** create sibling files like `…_PLAN.md` next to `…_CONCEPT.md` for the same subject.
+
+---
+
+## 1. One concept = one file
+
+Before creating **any** new markdown file under `docs/`:
+
+1. Search existing paths and titles for **similar names** or **overlapping topics**.
+2. If overlap exists → **update the existing file** (amend in place).
+3. Only create a new file when the topic is genuinely new and no current file owns it.
+
+## 2. Naming — one canonical name per topic
+
+- Pick a **single** canonical filename per subject and stick to it.
+- Do **not** create variants that differ mainly by role suffix, including but not limited to: `PLAN`, `CONCEPT`, `DRAFT`, `NOTES`, `OUTLINE`, `BRIEF`, `SPEC`, `V2`.
+- Bad: `SELF_SERVE_AI_DISCOVERABILITY_PLAN.md` + `SELF_SERVE_AI_DISCOVERABILITY_CONCEPT.md`.
+- Good: one `SELF_SERVE_AI_DISCOVERABILITY_CONCEPT.md` that holds both concept and phased plan.
+
+## 3. “Write a plan / concept / brief” means amend
+
+If a task says write a plan, concept, brief, or execution outline for something that **already has a file**, **amend that file**. Never add a sibling with a near-identical stem.
+
+Current example SoT: [`SELF_SERVE_AI_DISCOVERABILITY_CONCEPT.md`](./SELF_SERVE_AI_DISCOVERABILITY_CONCEPT.md).
+
+## 4. Deletes and consolidations
+
+- **Deleting** a docs file requires an **explicit** instruction (user or locked task).
+- Consolidations: **merge** unique content into the surviving file, **then** delete the duplicate(s). Do not leave two near-names “for later.”
+
+## 5. Announce path before create
+
+Every new docs file must be **announced with its exact path** before creation (in the working chat/call). If the announced path overlaps an existing topic, stop and amend instead.
+
+---
+
+## Quick checklist (agents)
+
+- [ ] Grepped / listed `docs/` for similar stems and topics  
+- [ ] Confirmed no PLAN/CONCEPT/DRAFT twin for the same subject  
+- [ ] Amending existing SoT when topic already exists  
+- [ ] Exact path announced before any `docs/` create  
+- [ ] Consolidation ends with a single surviving file  
+
+---
+
+## Automation note
+
+As of 2026-10-01 this repo has **no** Gitea Actions / `.gitea` / `.github` CI and only stock `.git/hooks/*.sample` files. Guardrails are **documented here** until CI or an active pre-commit hook is added. Prefer enforcing via this file + review rather than inventing hook infrastructure without a project standard.
+
+---
+
+## Capture
+
+Added after consolidating the self-serve AI discoverability PLAN+CONCEPT duplicate on `planning-selfserve-audit`.