Validate config.yml on Server Startup

[adarshm11]

Summary

config.yml is currently loaded without any validation. Missing or malformed fields silently cause repos to misbehave or produce cryptic runtime errors during a deployment. We should validate the config at startup and fail fast with a clear error message.

Problem

If a user adds a new repo entry to config.yml and forgets a required field (e.g. branch, path, or compose_file), the server starts without complaint. The error only surfaces when the first webhook arrives — often in the middle of a deployment — making it hard to debug.

Proposed Solution

Add a validate_config(config: dict) function that runs once at startup before the server begins accepting requests. It should:

1. Check that each repo entry has all required fields: name, path, branch.
2. Verify that each path exists on disk.
3. Warn (but don't fail) for unknown/unrecognized keys that might be typos (e.g. brach instead of branch).
4. Print a clear, actionable error and exit with a non-zero code on any validation failure.

REQUIRED_REPO_FIELDS = {"name", "path", "branch"}

def validate_config(config):
    for repo in config.get("repos", []):
        missing = REQUIRED_REPO_FIELDS - repo.keys()
        if missing:
            raise SystemExit(f"[config] Repo '{repo.get('name', '?')}' is missing fields: {missing}")
        if not os.path.isdir(repo["path"]):
            raise SystemExit(f"[config] Path does not exist for repo '{repo['name']}': {repo['path']}")

Acceptance Criteria

• Server exits with a descriptive error if any required field is missing from a repo entry
• Server exits with a descriptive error if a configured path does not exist on disk
• Unknown top-level keys produce a warning log but do not exit
• All existing valid config files continue to work without changes