fix: reuse current location for directory browsing

[opencode-agent]

Summary

Directory completion and the project picker used each directory being browsed as a new Location. The first request to each one booted its complete runtime, including inherited MCP servers.

Keep the existing filesystem endpoint and browse through one stable Location instead:

• Allow fs.list({ location, path }) to accept absolute paths or paths relative to the Location, including parents and siblings outside its cwd. Returned entries stay relative to that Location, including ./ when listing the cwd from its parent.
• Leave file-read containment, including symlink checks, unchanged.
• Keep the current Location and workspace selector during TUI /cd completion and app picker navigation, preloading, and suggestions. Only committed navigation changes the selected Location.
• Pass absolute listing paths directly from the app and TUI; no client-side relative-path calculation is needed.
• Pass the composer's Location into the picker only for the same server; Home and cross-server pickers resolve one server-default Location.
• Cover Desktop's shared remote/SSH/WSL picker. Local native Electron dialogs already browse without making filesystem API requests and need no IPC change.
• Preserve indexed recursive suggestions within the current Location; outside it, use direct directory listings instead of booting candidate runtimes.

No new endpoint, host-global filesystem fallback, or MCP lifecycle redesign.

Fixes #46826.

Validation

• Core filesystem tests: 35 passed, including parent/sibling listings, self-entry paths, literal file reads, and lexical/symlink read containment.
• Server filesystem + HTTP tests: 13 passed. The regression boots one real local MCP process, browses parents/siblings with relative and absolute paths, checks identical entries, and asserts that loaded Locations and MCP starts remain at one.
• App selection/workspace tests: 35 passed, including workspace identity, relative results, stale searches, drive/UNC paths, and literal ~ filenames.
• TUI prompt/location tests: 66 passed.
• Desktop file adapter/attachment and WSL connection tests: 14 passed.
• Core, Server, App, and TUI package typechecks passed. Regenerated the client with bun run generate; the Effect client's list path now accepts a plain string rather than requiring a RelativePath brand. Formatting and diff checks passed.
• Real Chromium app with a mocked filesystem: parent navigation, sibling expansion, and Escape passed. The same scenario on the base used four Location selectors; this branch used only /workspace/current across five listings.

Boundaries and follow-up

• Physical Windows validation was attempted but the shared Windows machine was unreachable over SSH. Native Windows/Electron interaction is not claimed as tested.
• The existing Path already exists: "sibling/nested/" tree-expansion error was reproduced on both the base and this branch. The navigation assertion passes here, but that separate preload/application defect remains unchanged.
• This retains the existing Location-scoped filesystem implementation and workspace identity; it does not implement the separate outstanding sandbox-filesystem routing work.

Requested by: @thdxr (dax via Slack)