fix(tasks): accept single-field task briefs

[dawNotPoi]

Fixes #698.

What changed

• Accept task drafts with a title, a description, or an attachment. A title-only brief becomes a real text prompt block, including when attachments are present.
• Derive a description-only task title from its first nonempty line (up to 80 characters) while retaining the full rich prompt blocks. Give attachment-only tasks a localized default title. Reject a fully empty brief.
• Persist optional brief_origin metadata so reopening a generated brief does not mistake its generated fields for user-entered text. Title-only edits rebuild the instruction from the current title; untouched attachment-only edits keep only attachments; description-derived titles follow the first line until the title is explicitly edited.
• Verify stored title, description, and attachment provenance against the actual config before using it. Inconsistent markers and legacy tasks with explicit text retain their stored fields. Legacy image-only tasks and templates retain image-only prompts.
• If the last attachment is removed from an untouched attachment-only brief, show a localized explanation and prevent submission. Editing the title explicitly still makes a valid title-only task.
• Keep template naming/upsert behavior and backend draft validation unchanged.

Compatibility

WorkTaskDraft.config and WorkTaskInfo.config are opaque serde_json::Value fields. The service stores and returns their JSON directly on create/get/update; template save/read does the same. The Rust WorkTaskConfig uses tolerant serde defaults and ignores the optional editor-only field during validation and launch. Older configs without the marker remain readable.

Verification

• Regression cases were written first and failed on the previous implementation, including reopen/edit, stale provenance, and last-attachment removal.
• Latest commit 51d51f42: focused task-editor suite 33 passed; i18n suite 19 passed; pnpm lint . passed; pnpm build passed.
• Full frontend suite on the earlier provenance commit 5ddc4ff3: 520 files, 7,790 tests passed. Latest CI on this head passed all 7 jobs (frontend and Rust server/desktop across Linux, Windows, and macOS): https://github.com/xintaofei/codeg/actions/runs/36038709483.

[dawNotPoi]

@xintaofei Ready for review. This fixes #698: task drafts now accept a title, description, or attachment alone. A title-only brief becomes a real Agent prompt; reopening and editing preserve the intended prompt, references, and attachments. Legacy image-only tasks and templates remain image-only, and removing the last attachment requires a real instruction. Independent review found and resolved the edit round-trip cases. All 7 latest CI jobs passed across frontend and Rust on Linux, Windows, and macOS; focused task-editor tests: 33 passed, i18n tests: 19 passed. Thanks!