[Feature Request] --resume should restore the last active model (including [1m] variants)

Summary

When resuming a session with --resume, the active model defaults to whatever is configured at startup rather than the model that was last used in the session. This breaks continuity, especially when:

• The user switched models mid-session (e.g. /model claude-opus-4-8[1m])
• The session used a [1m] context variant that affects prompt cache validity
• The session was long-running and the user expects to continue exactly where they left off

Expected Behavior

--resume should restore the last active model at the time the session ended — not the model at session start. If the user switched models during the session, the final model should be what gets restored.

This should include full model ID fidelity, including:

• Base model names (e.g. claude-sonnet-4-6)
• [1m] context window variants (e.g. claude-opus-4-8[1m])
• Any other modifier suffixes

Current Behavior

The active model reverts to the default or whatever --model is passed at startup. Mid-session model switches are silently lost.

Why "Last Used" Matters

A user may start a session with the default model, then switch to a more capable or larger-context model for heavy work. On resume, they expect to continue with that heavier model — not silently downgrade back to the default.

Implementation Suggestion

Persist the active model ID in the session journal on every model change (and at session close). On --resume, read the last persisted model from the journal and apply it before replaying context — unless --model is explicitly passed, which should take precedence.

Related Issues

• #65805 — --resume drops the [1m] modifier (invalidates prompt cache)
• #66005 — --resume drops the session's --effort level

Both are symptoms of the same root cause: session resume doesn't restore runtime state. This request asks for that to be fixed holistically for the active model.