Support tool discovery for vfox backend plugins

[3w36zj6]

Problem

A vfox backend plugin can manage multiple tools with the plugin:tool format and list versions once a tool name is known. However, it cannot tell mise which tool names the backend provides.

Users must already know the exact identifier, such as my-plugin:some-tool, and tools provided by a vfox backend plugin cannot be discovered through shell completion, mise search, or the interactive mise use selector. This becomes difficult to work with when a plugin provides many tools or its catalog changes over time.

Tools and aliases already included in the mise registry are discoverable regardless of their backend. However, built-in package backends do not generally enumerate every package in their upstream registries for completion. The limitation specific to vfox backend plugins is that they cannot opt into tool discovery even when the service behind the plugin has an authoritative tool catalog.

Proposed feature

Allow mise to discover the tools provided by a vfox backend plugin, including at least each tool's name and optionally a short description.

mise could use that information so users can:

• discover plugin:tool identifiers with mise search
• complete tool names after a backend plugin prefix
• find backend-provided tools in the interactive mise use selector

Existing plugins that do not provide a tool list should continue to work as they do today. This does not need to change how individual tool versions are listed or installed. Whether this should be a vfox-specific capability or a general backend discovery interface can be decided as part of the discussion.

[jdx]

Implemented in #13111.

The PR supports two optional runtime hooks so plugins can choose the right discovery model:

• BackendListTools(ctx) returns a small, finite catalog for no-query completion and the interactive mise use selector.
• BackendSearchTools({query}) performs query-driven discovery for large ecosystems such as npm, crates.io, or PyPI without enumerating the entire registry.

A plugin may implement either hook or both. mise prefixes returned names with the installed plugin name (plugin:tool), merges and deduplicates the results in mise search and shell completion, and routes plugin:query only to that plugin. Independent plugin lookups run concurrently. List results and per-query search results are cached using the remote-version cache duration and timeout, invalidated when the plugin changes, and can fall back to matching stale data if refresh fails.

Plugins without either hook behave as before, and mise registry remains the static registry-only command.

AI-assisted — Tool: Codex; model: unavailable; version: unavailable.

[3w36zj6]

Thanks for implementing this!