fix(tasks): fall back to settings.json api_key when spawning local agents

[sridhar-3009]

Summary

Closes #309

create_agent_task() raised ValueError ("Local agent tasks require
ANTHROPIC_API_KEY...") when a user's API key was configured in
settings.json but not exported as an environment variable. The check
only looked at the explicit api_key argument and os.environ.

Add a lazy fallback that reads settings.json before raising:

effective_api_key = api_key or os.environ.get("ANTHROPIC_API_KEY")
if not effective_api_key:
    from openharness.config.settings import load_settings
    effective_api_key = load_settings().api_key or None
if not effective_api_key:
    raise ValueError(...)

The import is lazy (inside the branch) to avoid any circular-import risk
at module load time.

Test plan

• Set api_key in ~/.openharness/settings.json, unset ANTHROPIC_API_KEY. Verify that spawning a local agent task succeeds.
• Verify that an explicit api_key argument still takes precedence over both env var and settings.
• Verify that ANTHROPIC_API_KEY still takes precedence over settings when both are set.
• Verify that the ValueError is still raised when none of the three sources provides a key.