[FEATURE] Allow custom/enterprise model IDs to appear in `/model` interactive picker

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When using enterprise or custom model IDs (e.g., IDs with non-standard prefixes
provided by enterprise API deployments), these models do not appear in the
interactive /model picker.

Users can still set them via /model <custom-model-id> or settings.json,
but they are excluded from the interactive selection list.

Proposed Solution
Add a customModels field to settings.json that allows users to register
additional model IDs for display in the /model picker:

Proposed Solution

Add a customModels field to settings.json that allows users to register
additional model IDs for display in the /model picker:

{
"customModels": [
"custom-model-id-sonnet",
"custom-model-id-opus"
]
}

Alternative Solutions

• When /model is invoked, query the API for accessible models and display

all that are available to the authenticated team/org.

• Allow inline search in the picker so users can type a partial model ID

and match custom models.

Priority

Critical - Blocking my work

Feature Category

CLI commands and flags

Use Case Example

Enterprise API deployments often provide model IDs with custom prefixes
(e.g., "XXXX-claude-4-6-sonnet-v1") that map to standard Claude models.
Switching between Sonnet and Opus variants is a common workflow, and the
current picker doesn't support them, adding unnecessary friction.

Additional Context

• Workaround: /model <full-model-id> works, but requires memorizing the

exact ID each time.

• The current model is saved in settings.json ("model" field), so the

infrastructure for custom IDs already exists.

Additional Context

_No response_