fix: fail loudly when Copilot agent files are not created

[Lucas-Frazao]

Problem

hsdd init could complete successfully while the .github/agents/ directory was never created. This happened because CopilotIntegration.setup() silently returned an empty list when it couldn't locate command templates (e.g. due to install path resolution differences between dev/wheel installs, Windows path quirks, or venv isolation).

The result: users run hsdd init, see success, open VS Code, type /hsdd.constitution — and nothing shows up.

Root Cause

_locate_commands_dir() checks two hardcoded paths:

1. core_pack/commands/ (wheel install)
2. ../../templates/commands/ relative to __file__ (dev install)

If neither resolves — which can happen with editable installs, symlinked venvs, or certain Windows configurations — list_command_templates() returns [], and setup() returns [] without any warning.

Changes

CopilotIntegration.setup()

• Raises FileNotFoundError when no command templates are found, with diagnostic info showing which paths were searched.

hsdd init

• Catches FileNotFoundError from the integration, shows a clear error with remediation steps (hsdd integrate copilot), and exits non-zero.
• Post-validates that .agent.md files actually exist on disk after setup() returns (defense in depth).

hsdd integrate

• Same error handling as init.

hsdd check

• Now reads init-options.json to determine the configured AI assistant.
• If ai_assistant is copilot but .github/agents/ is missing, flags it as an error (not a skip) with a remediation message.

Tests

• 9 new tests covering the Copilot integration setup:
  • Agent files created
  • Prompt files created
  • copilot-instructions.md created
  • Correct file extensions
  • Prompt files reference correct agent names
  • Raises on missing templates (the key regression test)
  • Idempotency
  • Constitution agent specifically exists
  • All 10 expected commands present

All 55 tests pass.