docs: add LOCAL_DEV and CONTRIBUTING guides, explain Delta7d

Two new short docs unblock new contributors who currently have to read
the Go source to figure out the GITHUB_TOKEN requirement and the
agents.yml schema:

- docs/LOCAL_DEV.md walks through the PAT setup, the local run command,
  what files the run modifies, and how to revert before opening a PR.
- docs/CONTRIBUTING.md documents the agents.yml fields, enumerates the
  six valid category values, and explains the rename and deprecation
  policy now that history is keyed canonically.

Also add a one-sentence caption under the table in readme.tmpl so the
Delta7d column has a definition in the rendered README.

docs/LOCAL_DEV.md

@@ -0,0 +1,50 @@
+# Local Development
+
+## Prerequisites
+
+- Go 1.23 or later
+- A GitHub personal access token (PAT)
+
+## Getting a GitHub Token
+
+1. Go to [https://github.com/settings/tokens](https://github.com/settings/tokens)
+2. Click "Generate new token" → "Generate new token (classic)"
+3. Give it a name (e.g., "awesome-coding-agents")
+4. Select scope: **`public_repo`** (needed to read public repo metadata)
+5. Click "Generate token" and copy the value
+
+The updater reads public repos only; `public_repo` scope is sufficient and safe.
+
+## Running Locally
+
+```bash
+export GITHUB_TOKEN=ghp_your_token_here
+go run .
+```
+
+The tool will:
+- Read `data/agents.yml`
+- Fetch live metadata from GitHub GraphQL API
+- Append star counts to `data/history.jsonl`
+- Regenerate `README.md` from `templates/readme.tmpl`
+
+## Reverting Local Changes
+
+Before opening a PR, undo local modifications:
+
+```bash
+git restore data/history.jsonl README.md
+```
+
+This removes the snapshot files generated by your local run, leaving only your edits to `data/agents.yml`.
+
+## Rate Limits
+
+- **With GITHUB_TOKEN:** 5,000 requests/hour
+- **Without token:** 60 requests/hour
+
+Always set `GITHUB_TOKEN` when testing locally to avoid hitting the unauthenticated limit.
+
+## Next Steps
+
+To add agents, see [CONTRIBUTING.md](./CONTRIBUTING.md).