fix: label image references with filenames

[kristofferremback]

Problem

Image references used generated labels such as Image #3, while the attachment tray showed filenames such as pasted-image-4.png. Removing an earlier image or inserting the same image twice left both labels factually valid but visually contradictory, and users had no visible numbered list to reconcile them against.

Solution

Image references now use the attachment filename everywhere users encounter the pointer: editor chips, plain-text copy, serialized markdown, sent-message links, and agent-authored attachment links. Images now follow the same naming rule as PDFs and other files, so a tray item and each reference to it share one stable label.

attachmentReferenceLabel in @threa/prosemirror owns the filename-first rule for both markdown and TipTap. Existing [Image #N](attachment:...) content still parses. When old content has metadata, it renders the stored filename; metadata-free legacy image links retain Image #N as a fallback. No migration or attachment ID behavior changes.

The companion prompt and design-system examples now document filename labels. Opus independently recommended this choice because filenames remain stable under tray mutation and give screen readers the same identifier visible elsewhere.

Verification

• 143 focused frontend tests, 109 markdown tests, 35 backend prompt tests, and 7/7 inline-upload browser tests; full monorepo typecheck and lint clean
• Browser coverage includes paste, mobile multi-pick, sent-message lightbox, hover linking, and duplicate copied references

Residual risk

imageIndex remains in contentJson and clipboard HTML for backward compatibility, but it no longer overrides an available filename.

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📥 Commits

Reviewing files that changed from the base of the PR and between 958443c and 462b83d.

📒 Files selected for processing (14)

• apps/backend/src/features/agents/companion/prompt/system-prompt.test.ts
• apps/backend/src/features/agents/companion/prompt/system-prompt.ts
• apps/backend/src/features/messaging/handlers.ts
• apps/frontend/src/components/editor/attachment-reference-extension.test.ts
• apps/frontend/src/components/editor/attachment-reference-extension.ts
• apps/frontend/src/components/editor/attachment-reference-view.tsx
• apps/frontend/src/components/editor/editor-markdown.test.ts
• apps/frontend/src/components/timeline/pending-attachments.tray-drag.test.tsx
• docs/design-system-kitchen-sink.html
• docs/design-system.md
• packages/prosemirror/src/index.ts
• packages/prosemirror/src/markdown.test.ts
• packages/prosemirror/src/markdown.ts
• tests/browser/inline-file-upload.spec.ts

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