feat: support Discord file attachments

[DrVictorChen]

Summary

• Discord users can now send files (PDFs, images, code, etc.) alongside messages
• Attachments are downloaded to /tmp/openab_attachments/{thread_id}/ and file paths are appended to the prompt text, so any ACP backend agent can read them with its native file tools — no ACP protocol changes needed

Related: #161 (feature request for multimodal support), #158 (image-only via ACP ImageContent blocks). This PR takes a complementary approach — downloading all file types to disk and injecting paths into the prompt, which works for PDFs, code files, and other non-image attachments without requiring ACP protocol changes. The two approaches (#158 for images, this PR for general files) could coexist.

How it works

User sends message + PDF in Discord
         │
         ▼
OpenAB downloads attachment to /tmp/openab_attachments/{thread_id}/paper.pdf
         │
         ▼
Prompt becomes:
  "Please summarize this paper
  
   Attached files:
   - /tmp/openab_attachments/123456/paper.pdf (application/pdf, 2.3 MB)"
         │
         ▼
ACP agent reads the file path with its own file read tool

Changes

File	Change
Cargo.toml	Add reqwest (rustls-tls, matching serenity's TLS backend)
src/discord.rs	Add download_attachments(), inject paths into prompt, allow attachment-only messages
src/acp/pool.rs	Clean up temp attachment directory when sessions are garbage-collected

Safety

• Filenames sanitized (only alphanumeric, ., -, _)
• Max 25 MB per file, max 5 attachments per message
• Download failure → graceful fallback (includes Discord CDN URL instead)
• Temp files cleaned up on session cleanup

Test plan

• cargo build --release passes
• Send text-only message → behavior unchanged
• Send PDF attachment → agent receives file path and can read content
• Send attachment without text → prompt contains only attachment info (not rejected as empty)
• Send file >25 MB → skipped with friendly message
• Session cleanup → /tmp/openab_attachments/{thread_id}/ removed

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