Add Obsidian agent plugin

[christophertjiattas]

Summary

Adds a built-in obsidian-agent plugin that registers a specialized agent for working with Obsidian vaults through the official obsidian CLI.

The agent focuses on safe vault workflows:

• uses the Obsidian CLI for vault-aware operations
• discovers before modifying whenever practical
• reads before writing note content
• asks for explicit confirmation before destructive, broad, or hard-to-reverse operations
• includes troubleshooting guidance for common CLI setup issues

Implementation notes

• Implements the agent as a plugin under code_puppy/plugins/obsidian_agent/ using the existing register_agents callback hook.
• Keeps the agent tool surface intentionally small: shell execution plus structured user confirmation.
• Includes plugin-local documentation in code_puppy/plugins/obsidian_agent/README.md.
• Avoids user-specific defaults, vault paths, note names, or private configuration.

Testing

Ran:

uv run ruff format code_puppy/plugins/obsidian_agent tests/plugins/test_obsidian_agent.py
uv run ruff check code_puppy/plugins/obsidian_agent tests/plugins/test_obsidian_agent.py
uv run pytest tests/plugins/test_obsidian_agent.py
uv run python - <<'PY'
from code_puppy.plugins import load_plugin_callbacks
from code_puppy.agents.agent_manager import get_available_agents, refresh_agents

load_plugin_callbacks()
refresh_agents()
agents = get_available_agents()
assert 'obsidian-agent' in agents, agents.keys()
print('obsidian-agent:', agents['obsidian-agent'])
PY

Also scanned the new plugin and tests for local/private path markers before opening the PR.