feat(executor): wire the agent MCP back-channel for gemini containers (Phase 4)

ContainerRunner previously skipped the MCP back-channel for gemini agents, so
they ran tool-less. It now mints a per-task token for gemini too and registers
the agent MCP server in the gemini CLI's user settings
(agentHome/.gemini/settings.json → $HOME/.gemini in-container) using the
gemini-cli mcpServers/httpUrl schema with a bearer header. With MCP enabled,
gemini also receives the planning preamble that points at the ask_user/
report_summary/spawn_subtask/record_progress tools.

Config generation is unit-tested (TestWriteGeminiMCPSettings) and the write is
in the run setup path. CAVEAT: whether the gemini CLI actually invokes these
tools in non-interactive (-p) mode — and how it handles tool auto-approval —
is NOT verified against a live gemini binary in this environment; this lands
the plumbing for a follow-up spike. No regression risk for gemini runs: a
config issue degrades to the prior tool-less behavior.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

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