feat(spawn): per-task crewmate model selection for cost control

[JYoussouf]

Problem

bin/fm-spawn.sh launches every crewmate as plain claude --dangerously-skip-permissions with no --model flag, so all crewmates inherit the CLI default (Opus). Trivial, mechanical tasks (em-dash sweeps, file gathering, simple find-and-replace) run on the most expensive model with no way to downshift.

Proposal

Add per-task model selection to fm-spawn:

• Accept an optional --model <name> (e.g. haiku|sonnet|opus, or any value the harness accepts).
• For the claude harness, thread it into the launch as --model <name>; keep other harnesses working (default/ignore if no per-harness flag yet).
• Record model= in state/<id>.meta; omitting --model preserves today's exact behavior (inherit CLI default).
• Document a tier policy in AGENTS.md: Haiku = trivial mechanical work; Sonnet = straightforward edits/scouts/integration; Opus = complex/judgment-heavy/integrator.

Why

Lets firstmate route work to the cheapest capable model per dispatch, cutting cost on the long tail of simple tasks while reserving Opus for hard work.

[indi256s]

Closing as completed in current main.

Evidence checked:

• bin/fm-spawn.sh accepts per-spawn model and effort axes and records them in task metadata.
• AGENTS.md documents profile selection and harness-specific model/effort behavior.
• docs/examples/crew-dispatch.json includes cheap/mechanical and heavy-work routing examples.
• tests/fm-spawn-dispatch-profile.test.sh covers the launch flag behavior across supported harnesses.