Custom agent frontmatter: accept array for `model:` field

[dknappettmsft]

Feature request

Allow custom agent markdown frontmatter to accept an array of strings for the model: field, in addition to a single string.

Motivation

VS Code chat modes already support model: as a list — it's used to present a model picker for the chat mode. This means the same agent definition file can't currently be shared cleanly between VS Code and Copilot CLI: the CLI rejects the file with:

custom agent markdown frontmatter is malformed: model: Expected string, received array

This forces plugin authors to either:

• Maintain two copies of every agent file (one for VS Code, one for CLI), or
• Pick one tool and accept that the agent won't load in the other.

Proposed behavior

When model: is an array, the CLI uses the first entry as the default model for that agent (and ideally treats the rest as acceptable alternates the user can switch to with /model while invoking the agent). When model: is a string, behavior is unchanged.

Alternatives considered

• Authors flatten model: to a single string — works in both, but loses the picker UX in VS Code.
• A local post-install patch script — fragile, breaks after every plugin update.

Accepting an array in the CLI parser is the only change that lets the same plugin work unmodified in both surfaces.