fix: make spawned subagents follow the parent session's model

[SergioGen]

Problem

Subagents spawned via the spawn_agent tool create sessions through session.prompt without passing a model, so they fall back to the model baked into the agent config at plugin startup. Switching models with /models only affects the running session, so subagents keep using the stale model - after a rate limit they error out even though the user switched models.

Fix

• Capture the live model from each chat.message into a per-session map
• Inject it into the subagent session's session.prompt body, so subagents follow the parent session's model
• Propagate the model to child sessions so nested spawns (e.g. executor -> implementer) keep following it
• Clean up the map entry on session.deleted

Changes

• src/session-model.ts (new): in-memory session model map with get/set/delete accessors
• src/index.ts: capture model in chat.message, clean up on session.deleted
• src/tools/spawn-agent.ts: inject and propagate the parent session model
• tests/session-model.test.ts (new): 3 unit tests for the map helpers

Verification

• tsc --noEmit: pass
• eslint: pass
• biome check: pass
• bun test: not run locally (no bun available) - CI should run it

---

Summary by cubic

Spawned subagents now follow the parent session's active model instead of falling back to the model baked in at plugin startup, which caused rate-limit errors after switching with /models.

• Capture the active model from each chat.message, replay it onto spawn_agent child sessions, and propagate it so nested spawns keep following it.
• Agents with an explicit model in micode.json keep it—their override wins over the parent session's live model.
• Delete the stored model on session.deleted.
• Add src/session-model.ts and unit tests for the session-model map helpers.

^{Written for commit c0323ab. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed

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