fix(cli): handle writeFileSync errors in export --out with a clean message

[Open2Loop]

Summary

Fixes #69.

agentctx export --out <path> called writeFileSync with no error handling. When the target directory doesn't exist (or the path isn't writable), the raw Node stack trace was printed instead of a clean error message, because reportError in main.ts only recognises structured error types.

Fix: wrap writeFileSync in a try/catch that emits a single-line agentctx export: <message> error on stderr and returns exit code 1 — matching how other filesystem failures are handled in the CLI.

Change

// Before
writeFileSync(values.out, markdown, "utf8");
env.io.out(`✓ exported ${count} record(s) to ${values.out}`);

// After
try {
  writeFileSync(values.out, markdown, "utf8");
} catch (err) {
  const msg = err instanceof Error ? err.message : String(err);
  env.io.err(`agentctx export: ${msg}`);
  return 1;
}
env.io.out(`✓ exported ${count} record(s) to ${values.out}`);

🤖 Generated with Claude Code

[Deepam02]

@Open2Loop please add the tests as required by the issue description.