Add MCP CommandPlan preview without execution

[Eilen6316]

Goal

Allow MCP clients to request a CommandPlan preview while keeping execution unavailable over MCP.

Context

The MCP server is intentionally read-only in v4.1.0. A preview-only CommandPlan tool would let external agent clients inspect LinuxAgent's planning and policy boundary without creating a remote execution path.

Scope

• Add a MCP tool that returns a validated CommandPlan preview.
• Run policy classification for each proposed command.
• Include HITL requirement, sandbox profile metadata, and matched policy rules.
• Return structured content suitable for client display.
• Audit only if the operation records a formal proposal event; do not record fake execution.

Non-goals

• Do not execute commands.
• Do not apply file patches.
• Do not create background approvals or non-interactive auto-approval.
• Do not bypass existing planner validation.

Acceptance Criteria

• MCP preview returns validated plan data and per-command policy decisions.
• Unsafe or invalid planner output is rejected with structured errors.
• No subprocess or SSH execution path is reachable from this tool.
• Unit tests prove preview-only behavior.
• make lint, make type, make security, make red-team, and make test pass.