fix(session): keep temporary-task attachments readable after branching

[yuxino]

Pasting long text into a temporary task saves a file, but clicking the sent attachment shows “Open a project to browse files.” After Branch from here, the copied attachment instead reports “No file matches”: its path still belongs to the source task.

This makes explicit attachment previews work without a project and gives each branch its own copy of the referenced pasted/imported inputs. Deleting the original task then leaves the branch attachment readable.

Native macOS recording, before and after (53 seconds, 1920×1440). English UI and step annotations, highlighted results, 1–2× playback with idle waits trimmed. These are isolated builds of the real desktop app using a localhost model fixture; no UI or file-read results are mocked.

attachment-fix-before-after-hd.mp4

The production change is limited to FilesTab, the existing fork operation, and its file-copy helper. Only referenced existing files under scratch/<session>/pasted/ are copied; unrelated scratch outputs are excluded. Failed forks clean up their copies, and already-expired files remain unavailable. No cross-session read permission, IPC, or database schema change. ADR 0023 and the corresponding English/Chinese specs and E2E scenario reflect the attachment exception to scratch isolation.

Regression coverage checks temporary preview/back navigation, bounded and repeated forks, retained checkpoint paths, source deletion, and rollback. Both original failures were demonstrated before the fixes.

Validation details

• Candidate: 8ffb30e70cce373857a736a6549233feb806c714.
• Base: 0111e306c120ad5820688d7608cb37bad8fbcc1f; upstream, fork, and local main matched before publication.
• GitHub merge candidate c2c03705ce1c207544a7d0f9a110abbf04a259a6 has the same tree as the tested head (0a79f39a7bc3904e5aa3258f16ac497e4a837d93).
• macOS arm64, Node 24.13.0, pnpm 10.34.5.
• node scripts/e2e-composer-paste.mjs passed on the final candidate with rebuilt preload, including saved-byte verification and temporary-task preview/back.
• cargo test -p host-core --locked: 583 passed; Desktop tests: 2523 passed. Final commit adds only documentation to the tested Rust tree.
• JS build, Desktop typecheck, lint, cargo fmt --check, docs/locale checks, and PR-base check passed. Clippy completed with the existing user_skills.rs:921 warning.
• Earlier Electron suite attempts intermittently failed at the existing image-preview Tab-focus or pending-paste/session-switch assertions. The unmodified main and final candidate both passed sequential comparison runs. No assertions were weakened; the intermittent failure cause is not established.
• The native recording verifies the actual attachment workflow and source deletion. A behavior-preserving traversal cleanup and an additional rollback test were validated separately by the final Rust suite. Windows/Linux native validation was not run.