feat(bot-mode): keep Group Chat files durable on gateways

[dokterdok]

The user problem

Bot Group Chats can already discuss text after Desktop closes, but a shared
file does not yet have the same continuity. A screenshot, PDF, spreadsheet, or
document needs to remain available to the Bots that were in the chat when it
was sent, survive a gateway restart, and never leak a host path into history.

This is the durable gateway layer for that experience.

Important

This draft is stacked on #96654. Review a00706b742..57962642d7: one
commit defines the private attachment ledger and bounded task contract; the
second stages those files atomically into same-gateway Bot turns.

What it enables

When a file is shared with a gateway-hosted Group Chat:

1. Hermes stores the bytes privately on the Group Chat's gateway.
2. The room log keeps only an opaque ID and safe metadata.
3. Each Bot that belonged to the chat at send time receives the verified file
   through the same image, PDF, and file tools used by one-to-one chats.
4. The bytes and recipient snapshot survive restart and replay.

The result is simple from the user's perspective: files stay with the Group
Chat and the intended Bots can keep working with them after Desktop closes.

Safety and reliability

• Images and PDFs must match their declared content. Other file types remain
  opaque, so DOCX, XLSX, PPTX, EPUB, JAR, APK, and similar ZIP containers keep
  working.
• One message accepts up to 8 files, 15MB each, and 25MB total.
• One Bot task stages at most 16 files and 50MB. Larger backlogs advance in
  deterministic batches instead of retrying the same oversized task forever.
• Staging is attempt-scoped. If the second file fails, the first is rolled back
  and cannot leak into the next unrelated turn.
• Generic files created by a failed attempt are removed; pre-existing files are
  preserved.
• A late-added Bot does not gain access to historical files.
• Gateway and room quotas, abandoned-upload expiry, disband grace, SHA-256
  verification, private permissions, and symlink rejection are enforced.
• Room events contain no bytes, data URLs, local paths, or per-Bot transport
  references.

Review boundaries

• Same-gateway Bot delivery only in this draft.
• Desktop picker, paste, drag/drop, messaging ingestion, history rendering, and
  classic Desktop-driven Group Chats are the stacked surface follow-up.
• Cross-gateway RoomLink byte transfer is a separate increment with its own
  grants, receipts, and failure matrix.
• This does not change feat(desktop): group-chat attachment chips open in the file manager #89758's attachment-chip interaction.

Validation

Focused exact-stack validation:

Check	Result
Combined gateway/runtime attachment suite	included in 242 passing Python tests
Atomic staging rollback tests	passed
200-file backlog stress test	13 bounded, lossless batches
DOCX/XLSX/PPTX/EPUB/JAR/APK compatibility cases	passed
Ruff	passed
git diff --check	passed

The parent and child heads remain draft until exact-head live UAT is added.

Related work

• feat(desktop): group-chat bots now see user-attached images #89486: merged image attachment pipeline for Desktop Group Chats
• feat(desktop): group chats gain PDF/file attachments and drag & drop (#88983) #89540: merged PDF/file and drag/drop parity
• feat(desktop): group-chat attachment chips open in the file manager #89758: open attachment-chip interaction follow-up
• feat(bot-mode): add durable authority and replay for hosted rooms #95314 / feat(bot-mode): keep same-gateway rooms running without Desktop #95622: durable gateway-owned Group Chats
• feat(bot-mode): control hosted rooms from messaging chats #96274 / feat(bot-mode): control Desktop-driven Group Chats from messaging #96654: messaging control across hosted and Desktop-driven chats

Type of change

• Bug fix
• New feature
• Security fix
• Tests
• Refactor

[dokterdok]

Closing this cumulative prototype now that #97681 captures the user contract, safety boundaries, and phased landing plan against the rebuilt Bot Mode tree. Its useful layer can be re-cut as a narrow current-main contribution after the foundation direction is agreed.