test(spike): manual agent-MCP validation harness + Phase 2 validation findings

Adds cmd/spike, a manual (never run by CI) harness that starts the agent MCP
server on the host and points a real agent CLI at it to confirm tool calls reach
the AgentChannel. Validates Phase 2 end-to-end without Docker.

Findings against real claude 2.1.150 (sonnet-4-6):
  - PASS: claude discovers the server via --mcp-config (streamable HTTP + bearer)
    and calls record_progress + report_summary; both reach the AgentChannel,
    zero permission denials.
  - PASS: claude calls ask_user, gets the ErrAgentBlocked "end your turn"
    response, and ends its turn cleanly (human-in-the-loop path).
  - PRODUCTION NOTE: claude rejects --permission-mode bypassPermissions under
    root — the in-container agent must run non-root (ContainerRunner sets
    --user to the host uid), or tools must be pre-allowed via --allowedTools.
  - GEMINI spike could not run (no gemini binary); the harness's gemini path
    writes the production mcpServers/httpUrl settings and is ready to run where
    a binary exists.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

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