Implement ExecutionEnvironment trait for fcctl-core #666

Summary

Implement ExecutionEnvironment trait as required by terraphim_rlm crate.

Context

PR #426 (feat(rlm): RLM orchestration with MCP tools) requires the fcctl-core crate to provide an ExecutionEnvironment trait that VmManager can implement.

Requirements

The trait should define methods for:

• execute_code(&self, code: &str, ctx: &ExecutionContext) -> Result<ExecutionResult, Error>
• execute_command(&self, cmd: &str, ctx: &ExecutionContext) -> Result<ExecutionResult, Error>
• create_snapshot(&self, session_id: &SessionId, name: &str) -> Result<SnapshotId, Error>
• restore_snapshot(&self, id: &SnapshotId) -> Result<(), Error>
• list_snapshots(&self, session_id: &SessionId) -> Result<Vec<SnapshotId>, Error>
• delete_snapshot(&self, id: &SnapshotId) -> Result<(), Error>
• delete_session_snapshots(&self, session_id: &SessionId) -> Result<(), Error>
• capabilities(&self) -> &[Capability]
• backend_type(&self) -> BackendType
• health_check(&self) -> Result<bool, Error>
• cleanup(&self) -> Result<(), Error>

Related

Blocked by: #664 (Create fcctl-core crate)