Emit `codeguard-reviewer` subagent for Copilot / VS Code

[thomas-bartlett]

The codeguard-reviewer subagent currently only emits for Claude and
Cursor (see AGENT_HOSTS in src/artifact_targets.py). VS Code Copilot
users get the CodeGuard instructions but not the reviewer agent.

VS Code loads project subagents from .github/agents/*.agent.md. Note
the current emitter in src/emit_agents.py hardcodes the output filename
as <name>.md (L79), so it can't produce the .agent.md extension
as-is.

Test against a real VS Code Copilot install:

• Agent is discovered (appears in the host's agent list/picker)
• Agent is invocable and runs to completion on a small repo
• Agent can read the rule files at the configured path
• SARIF output is produced

Docs: https://code.visualstudio.com/docs/copilot/customization/custom-agents