Run MiniMax Code Subagents in agent-host views

[SaladDay]

MiniMax Code Subagents now run in agent-host views. This is the first caller of ViewSession.Spawn (#446).

• The reader runs in the view. MiniMax Code settles Subagents by reading its native history with node subagent-snapshot.mjs while the CLI runs. In a view, the adapter starts that reader through ViewSession.Spawn. It uses the closure's node and the data directory as the view presents it, in a process group of its own, so the reader runs with the Harness's isolation. The view's rejection of Subagents is deleted.
• Both modes share one reader path. Host mode keeps the installed reader. It now starts through clirunner, as the view mode does, and its stderr is drained.
• The host check moved. The check that a host reader is installed moved from execution validation into host option preparation, under the same condition, so a view needs no reader on the host.

Testing: go vet and -race for mcode.

MiniMax Code qualification row 9 (Subagents in a view) passes. The Subagent answer was SUBAGENT=NEEDLE, and the reader runs took 115–164 ms.

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