chore: update coder/coder dependency to v2.29.3

Updates github.com/coder/coder/v2 from v2.10.1 to v2.29.3 to resolve
security policy conflicts for users building custom images.

Also updates:
- tailscale.com replace directive to match coder/coder's fork
- agentsdk.New API usage (now uses WithFixedToken)
- Test imports and mock interface implementations

Fixes #147

Author: kacpersaw