fix(heartbeat): include file path in JSON parse errors

[oratis]

Closes #5.

What

loadHeartbeatConfig was the only user-config loader of the three that didn't tag JSON parse failures with the offending file path:

Loader	Path	Error wrap
mcp/config.ts	~/.lisa/mcp.json	✅
channels/config.ts	~/.lisa/channels.json	✅
heartbeat/config.ts	~/.lisa/heartbeat.json	❌ (bare `JSON.parse`)

A user with a typo in heartbeat.json got SyntaxError: Unexpected token … pointing at compiled dist/heartbeat/config.js with no hint that the problem was in their own config.

Change

Match the existing sibling pattern (same wording, same shape):

let parsed: HeartbeatConfig;
try {
  parsed = JSON.parse(raw) as HeartbeatConfig;
} catch (err) {
  throw new Error(`failed to parse ${FILE}: ${(err as Error).message}`);
}

Scope

• One file, one function (6-line diff)
• No public API change, no behavior change for valid configs, no new deps
• Only the failure-path message gets better

npm run typecheck clean.

🤖 Generated with Claude Code