feat(spawn): add --model flag for per-session model override

[exuan]

What & why

ao spawn had no way to set the model for a single session — the model
was resolved only from project/role agentConfig.model, so every session
in a project shared one model. This adds a --model <name> flag to
ao spawn that overrides the model for that one spawn.

Resolution precedence: --model > role agentConfig.model > project
agentConfig.model. The agent plugins already forward the resolved
config.model to the underlying CLI as --model, so no plugin changes
are needed; this PR only wires the spawn flag through to
resolveAgentSelection.

Changes

• --model <name> option on ao spawn
• SessionSpawnConfig.model threaded to resolveAgentSelection({ spawnModelOverride })
• spawnModelOverride takes precedence over role/project model in resolveAgentSelection

Backward compatibility

When --model is omitted, model resolution is unchanged.

How to test

ao spawn <issue> --agent claude-code --model claude-opus-4-7
ao spawn --help   # lists the new --model flag

• pnpm build and pnpm typecheck pass for core and cli
• ao spawn --help shows the --model flag

Issue

Addresses the per-session (ao spawn --model) part of #2116.