refactor: enhance encryption handling and error reporting in agent re…

[pikann]

Summary

This PR fixes several production-environment issues in the AI agent service: encryption key misconfiguration, repo tools unavailability in production containers, and silent swallowing of agent errors.

Changes

Fix encryption key propagation (deploy/docker-compose.prod.yml, scripts/install.sh)

• Added ENCRYPTION_KEY to the ai-agent service in the production docker-compose — without it, the service couldn't decrypt LLM API keys stored encrypted by the API service.
• Fixed the install script to generate a proper 64-character hex key (32 bytes for AES-256), up from 32 characters (16 bytes).

Fix repo tools injection in production (docker_workspace.py)

• In development, /app is bind-mounted from the host into the agent container, so sibling sandbox containers could share it via a volume mount. In production, /app is baked into the image and the host path detection fails.
• Added _copy_repo_tools_to_container() which uses Docker's put_archive API to inject repo_tools.py and the required __init__.py files directly into the running sandbox container at /tmp/paca_tools when no bind-mount is detected.
• OH_EXTRA_PYTHON_PATH is now always set — to /app (dev) or /tmp/paca_tools (prod).
• Removed the MCP build directory (/mcp) volume sharing, which is no longer needed.

Improve error reporting (executor.py)

• _wait_for_done_or_stop now returns (stopped, errored) instead of a single bool.
• Conversations ending with ERROR or STUCK status now correctly set the conversation to "failed" and emit agent.conversation.failed, instead of being reported as finished.
• Added _get_conversation_error_detail() to extract and log the ConversationErrorEvent detail for observability.
• Fixed visualizer=_QuietVisualizer → visualizer=_QuietVisualizer() (was passing the class, not an instance).

Improve decryption failure handling (agent_repository.py)

• On decryption failure, return "" instead of the raw ciphertext. Forwarding ciphertext to the LLM provider produced misleading "token expired / incorrect key" errors; an empty key surfaces a clear "missing API key" error instead.
• Added an explicit warning when ENCRYPTION_KEY is unset, pointing to the fix.