Bug

When passing a model alias (e.g. gpt, sonnet, gemini-pro) to sessions_spawn's model parameter, the alias is not resolved to the full provider/model ref. The subagent silently falls back to the default model.

Steps to Reproduce

1. Configure aliases in agents.defaults.models:

   "openai-codex/gpt-5.4": {"alias": "gpt"}

2. Call sessions_spawn with model: "gpt"
3. The subagent runs on the default/fallback model, not GPT

Expected Behavior

The alias should resolve to the full model ref (openai-codex/gpt-5.4) before patching the child session, consistent with how /model gpt works in slash commands.

Root Cause

normalizeModelSelection() in reply-*.js just trims the string and returns it. It does not perform a reverse lookup against the agents.defaults.models alias map. Since the trimmed alias string is truthy, it short-circuits the rest of the resolution chain in resolveSubagentSpawnModelSelection().

The patched session receives a raw alias string that the gateway cannot match to a real provider/model pair, so it falls back to the default.

Workaround

Pass full model refs instead of aliases:

• openai-codex/gpt-5.4 instead of gpt
• anthropic/claude-sonnet-4-5 instead of sonnet
• google-gemini-cli/gemini-3-pro-preview instead of gemini-pro

Suggested Fix

Add reverse alias resolution in normalizeModelSelection() or resolveSubagentSpawnModelSelection() before returning the model string. The alias map is already available in cfg.agents.defaults.models.