Let @ mentions autocomplete paths outside the project (~/, ../, absolute)

[sergical]

Problem

The composer @ search covers only the current project (the thread worktree, or the project root). I often want to point the agent at a file outside it: a sibling repo, a config in ~/.config, a script in ~/bin, or a file one level up.

Today, typing a path after @ does not work:

• @../foo: the leading . and / are removed from the query, so it searches for foo inside the project.
• @~/foo and @/abs/path: matched as text against project-relative paths, so usually nothing.

The only ways to reference those files now are to drop a folder on the composer (desktop, local only), attach the file, or type the full path as plain text with no autocomplete.

Proposal

When the text after @ starts with ~/, /, ./ or ../, treat it as a path and autocomplete it like a shell, one segment at a time. Selecting an entry inserts the same file mention the folder drop already inserts (absolute path). Plain @foo keeps the current fuzzy project search.

The command palette already does this for folders (filesystem.browse, path detection in isFilesystemBrowseQuery), so the server-side path resolution exists.

Open questions

1. Files, not just folders. filesystem.browse returns directories only. This needs either a includeFiles option on browse or a separate call. Which do you prefer?
2. Remote environments. ~ resolves on the server's machine, which is right for "what the agent can read", but may surprise someone on a phone or on app.t3.codes. Is that acceptable, or should this be local-only at first like the folder drop?
3. Selecting a folder. Insert the folder as a mention, or continue the path (@~/src/ and keep listing)? I would continue the path, and insert on a second selection or a space.
4. Mobile. Mobile has its own @ search hook. Same change there in the same PR, or web first?

I would like to implement this if the direction and scope look right.

Investigated with Claude Code (Opus 5.5).