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/CONTRIBUTING.md

@@ -0,0 +1,50 @@
+# Contributing
+
+## Adding an Agent
+
+Edit [`data/agents.yml`](../data/agents.yml) and add an entry:
+
+```yaml
+agents:
+  - owner: github-username-or-org
+    repo: repository-name
+    category: cli
+```
+
+Required fields: `owner`, `repo`, `category`
+
+Optional fields: `notes` (for clarifications or caveats)
+
+## Field Reference
+
+| Field | Type | Required | Description |
+|-------|------|----------|-------------|
+| `owner` | string | Yes | GitHub user or organization that owns the repo |
+| `repo` | string | Yes | Repository name on GitHub |
+| `category` | string | Yes | One of the values listed below |
+| `notes` | string | No | Additional context or disclaimers |
+
+## Valid Categories
+
+- **cli** — Command-line tools and CLI wrappers
+- **ide** — Standalone editors and IDEs
+- **extension** — Editor extensions (VS Code, Neovim, etc.)
+- **library** — Libraries and SDKs
+- **research** — Research papers and proof-of-concept projects
+- **web** — Web-based tools and online IDEs
+
+## Handling Duplicates and Changes
+
+**Duplicate repos:** If a repo appears twice (same owner/repo), the second entry is ignored in the next daily run.
+
+**Renamed repos:** If a GitHub repo is renamed after being tracked, update the `repo` field in `data/agents.yml`. The updater will fetch fresh metadata under the new name. Historical data in `data/history.jsonl` remains under the old key and is not migrated.
+
+**Deprecation:** To remove an agent, delete its entry from `data/agents.yml`. The next run will drop it from the README; historical data is preserved.
+
+## PR Review
+
+- Keep PRs to changes in `data/agents.yml` only (do not edit `README.md` or `data/history.jsonl`)
+- The daily GitHub Actions workflow (runs at 00:00 UTC) picks up merged PRs automatically
+- No manual review required; the updater regenerates the README after your PR merges
+
+For local testing before opening a PR, see [LOCAL_DEV.md](./LOCAL_DEV.md).