File access parity: browse the whole sandbox, and show diffs on changed files in chat

[yuga-hashimoto]

Summary

Closes #330.

Two review-related gaps OpenCode already handled that Claude Code and Antigravity did not:

• Unrestricted file browsing — the workspace Explorer was locked to a single fixed workspace.path, so anything outside it (/root, /tmp, …) was invisible even though the app's own folder picker (RuntimeFolderBrowser/WorkspaceFolders) could already reach the whole sandbox rootfs and device storage. The Explorer now navigates freely up past the original root, reusing that same containment logic (WorkspaceFolders.hostDirectory) instead of ClaudeWorkspaceFiles's old two-root-only resolution.
• Changed files + diff in chat — the "file changes" card after a turn showed only file names, non-interactively, and nothing called the diff-capable sessionDiff/vcsDiff APIs that already existed for git-based backends. The card is now clickable and opens a diff dialog. A new RuntimeCapabilities.diffCapable flag (true for OpenCode local/remote and Claude Code, false for Antigravity) gates whether a diff is actually fetched — Antigravity and non-git workspaces show the file list with a "diff unavailable" message instead of a hard error (this also fixes a pre-existing bug where Antigravity's Explorer Changes tab threw an unrecognized error).

Changes

• ClaudeWorkspaceFiles now delegates root resolution to WorkspaceFolders.hostDirectory, gaining rootfs + device-storage browsing and adding synthetic /workspace (and device storage) entries when listing / so navigating up to the root is never a dead end.
• WorkspaceExplorerViewModel tracks the browsed directory as an absolute sandbox path instead of a workspace-relative one; navigateUp/open reuse WorkspaceFolders.parentOf/normalize. Search and git status/diff stay scoped to the original workspace on purpose.
• AntigravityTarget gains non-git-repo stub vcsInfo/vcsStatus/vcsDiff/sessionDiff (mirroring ClaudeCodeTarget's own fallback) and the shared Throwable.isNonGitWorkspaceError() extension (extracted out of WorkspaceExplorerViewModel).
• ChatViewModel gains openPatchDiff/dismissPatchDiff and PatchDiffState (Loading/Loaded/Unavailable); ChatPart.Patch now carries the owning message id so sessionDiff can be scoped to the turn.
• New ChatDiffDialog composable reuses WorkspaceExplorerScreen's existing UnifiedDiffView/SplitDiffView (now internal) rather than duplicating diff rendering.

Test plan

• ./gradlew :app:compileGithubDebugKotlin / :app:compileFdroidDebugKotlin — both flavors compile
• ./gradlew :app:testGithubDebugUnitTest — full suite green, 1105/1105 tests passing, including new/updated coverage for ClaudeWorkspaceFiles (rootfs + device storage browsing), WorkspaceExplorerViewModel (absolute-path navigation), WorkspaceFolders (synthetic root names), ChatViewModel (openPatchDiff/dismissPatchDiff), and backend diffCapable wiring
• Manual on-device check: browse a Claude Code workspace up past its root into /root, open a file outside the original root, confirm it loads; trigger a turn that edits a file and confirm the diff dialog on a git workspace, and the "diff unavailable" state on an Antigravity/non-git session — not done in this environment (no simulator/emulator available), flagging for manual verification

🤖 Generated with Claude Code