Bind agent mentions to visible source identities

**What problem would this solve?**

Agent autocomplete currently presents a name and description but not the source type or path. When project-local and global sources use the same name, discovery resolves the collision by order while the selected mention remains name-only. Users cannot reliably tell which source will run, and the client cannot preserve the identity of the item they selected.

**What would a good outcome look like?**

- Autocomplete visibly distinguishes project-local, global, recipe, and agent sources when names collide.
- Selecting an entry binds invocation to that exact source identity instead of repeating a name-only lookup.
- Existing unambiguous `@name` use remains compatible.
- Ambiguous or stale identities fail visibly rather than silently selecting another source.

**Possible approaches**

- Extend the ACP mention payload and invocation syntax with an opaque source identifier while retaining `@name` as display text.
- Define explicit collision behavior and provenance labels across server discovery, Desktop autocomplete, and CLI invocation.
- Alternatively, reject ambiguous duplicate names and require the user to rename one source.

The design should specify precedence, backward compatibility, how moved/deleted sources fail, and tests covering local/global collisions from discovery through invocation.

**Additional context**

This requires a cross-component identity contract, so implementation should wait for agreement on the intended UX and compatibility behavior.

- [x] I have verified this does not duplicate an existing feature request

Do not begin implementation until the issue reaches **Ready** on the [Goose Issues board](https://github.com/orgs/aaif-goose/projects/1).