fix(model-picker): list each provider/model combo separately (#970)

* fix(model-picker): list each provider/model combo separately

Previously, /model grouped models by name and showed all providers
that offer the same model (e.g. 'claude-sonnet-4-5 — anthropic, google-antigravity').
This was confusing because:
1. Users couldn't tell which provider would be used when selecting by number
2. The display implied choice between providers but selection was automatic

Now each provider/model combination is listed separately so users
can explicitly select the exact provider they want.

- Remove model grouping in buildModelPickerItems
- Display format changed from 'model — providers' to 'provider/model'
- pickProviderForModel now returns the single provider directly
- Updated tests to reflect new behavior

* fix: simplify model picker entries (#970) (thanks @mcinteerj)

---------

Co-authored-by: Keith the Silly Goose <keith@42bolton.macnet.nz>
Co-authored-by: Peter Steinberger <steipete@gmail.com>

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## 2026.1.15 (unreleased)
 
+- Fix: list model picker entries as provider/model pairs for explicit selection. (#970) — thanks @mcinteerj.
 - Daemon: fix profile-aware service label resolution (env-driven) and add coverage for launchd/systemd/schtasks. (#969) — thanks @bjesuiter.
 - Docs: clarify multi-gateway rescue bot guidance. (#969) — thanks @bjesuiter.
 - Fix: guard model fallback against undefined provider/model values. (#954) — thanks @roshanasingh4.