feat: Add update checker tooling

Add the initial dataset, version checker, tests, and project setup files so the update checker can be run and validated.

AGENTS.md

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+# Agents
+
+This project uses three cooperating agents to build the update checker.
+
+## Research agent
+
+Focus:
+- Identify `current_version_url` for each webservice.
+- Determine extraction rules for the running version (JSON path, regex, or text pattern).
+- Identify `upstream_latest_version_url` and extraction rules for the latest release.
+- Record notes for services without reliable version endpoints.
+
+## Coding agent
+
+Focus:
+- Define the YAML schema and validation rules.
+- Implement dataset loading and validation.
+- Implement version fetching and extraction based on dataset rules.
+- Implement version normalization, comparison, and output filtering.
+
+## Testing agent
+
+Focus:
+- Add unit tests for extraction helpers and version comparison.
+- Add fixtures for JSON/text/HTML response parsing.
+- Validate behavior for unknown or missing versions.