feat: multi-format file attachments with @-mentions and preview renderers

[OmGuptaIND]

Summary

Extend the composer beyond image-only attachments. Users can now upload documents (pdf, docx, doc, rtf), spreadsheets (xlsx, xls, csv, tsv), and text (md, txt, log, json) directly from the composer or Files view; reference any file or folder in the project via @ with shell-style drill-in; and preview uploads in the artifact panel (docx / xlsx / pdf / image). The harness translates markers to absolute-path <file_ref> / <dir_ref> XML so the agent can read / bash them on demand.

Spec: specs/features/MULTI_FORMAT_ATTACHMENTS.md
Follow-ups: specs/features/MULTI_FORMAT_ATTACHMENTS_FOLLOWUPS.md

UI

• Composer + splits into Add Images / Add Files
• DestinationPicker modal with folder tree + inline "New folder" + batch mode + smart mime-family defaults + recent-folder memory
• @ dropdown with files + folders, Finder-style drill-in (chevron / →), back-navigation row for parent scope, keyboard nav + Backspace to pop a level
• File / folder pills in composer with icon, hover-reveal remove (×), click to open artifact panel (file) or Files view (dir)
• All floating UIs portaled to document.body so ancestor transforms don't break fixed positioning
• Session Files bar splits into Uploads + Artifacts sections
• Sent messages render [img:id] / [file:path] / [dir:path] markers as clickable chips

Rendering

• Artifact panel gains DocxRenderer (mammoth), XlsxRenderer (SheetJS — with working "Show all rows"), PdfRenderer (<embed>), ImageRenderer — all lazy-loaded
• fs_read_bytes primitive for binary fetch, 500MB cap

Harness

• translateFileDirMarkers expands markers into XML refs with absolute paths and shallow folder listings (≤20 items)
• workspacePath threaded into buildInterleavedContent at both call sites

Infra

• cloud-init installs poppler-utils, pandoc, python3-pandas, python3-openpyxl, xlsx2csv so the agent can process uploaded docs via its shell / read tools
• Mirrored in deploy/Dockerfile

Protocol (additive)

• fs_list_response echoes requested path so concurrent listeners correlate correctly
• fs_read_bytes / fs_read_bytes_response for binary artifact previews

Test plan

• Composer + Add Images → pick PNG → inline chip (regression)
• Composer + Add Files → pick .md → DestinationPicker opens at notes/ → Save → pill in composer → Files view shows it under notes/
• Drag .xlsx onto composer → picker defaults to data/ → Save → pill → click pill → artifact panel renders sheet tabs + table
• Type @sal → dropdown → Enter to select → pill inserted
• Type @data/ → dropdown scopes into data/ → ← / Backspace returns to root
• Type @email (folder) → Right arrow or chevron → drills in; Enter selects the folder itself
• Hover a pill → × appears → click removes pill
• Send message with pills → harness log shows <file_ref name path /> / <dir_ref>
• Open artifact → xlsx with >5000 rows → "Show all" reveals remaining rows
• Drag multiple files → single picker → batch list → Save writes all
• Smart-default folder doesn't exist → picker shows friendly hint, Save still creates it
• Navigate away from a draft with pills → remount → pills restored
• Home (hero) composer → menu / dropdown / modal anchor correctly (transform-ancestor fix)
• Enter on a dropdown row selects — doesn't also send the half-typed @query

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