CS-10113: support AI Assistant file attachments

[lukemelia]

Summary

• add end-to-end AI Assistant file attachments from realm and local sources with pending/error/retry UX and send gating
• upload attachments to Matrix with FileDef representations, binary-safe content-hash dedupe, and stable attachment ordering
• make prompt construction model-aware (native image inputs where supported, metadata fallback for unsupported MIME) and scope tool-based file reads to same conversation

[github-actions]

Preview deployments

• Host staging preview
• Host production preview

[github-actions]

Host Test Results

    1 files  ±    0      1 suites  ±0   2h 42m 46s ⏱️ + 1h 7m 42s
1 643 tests  -   223  1 613 ✅  -   236  13 💤  -  2   2 ❌ + 2  15 🔥 +13 
3 312 runs  +1 431  3 230 ✅ +1 368  26 💤 +11  30 ❌ +28  26 🔥 +24 

For more details on these failures and errors, see this check.

Results for commit 9ae97fe. ± Comparison against base commit a49c7b3.

This pull request removes 226 and adds 3 tests. Note that renamed tests count towards both.

Chrome ‑ Acceptance | Code patches tests: LLM mode event controls auto-apply of code patches with timestamp checking
Chrome ‑ Acceptance | Code patches tests: automatic Accept All spinner appears in Act mode for multiple patches
Chrome ‑ Acceptance | Code patches tests: can create new files using the search/replace block
Chrome ‑ Acceptance | Code patches tests: can restore content of a patched file to its original state
Chrome ‑ Acceptance | Code patches tests: empty file shows generating content while streaming a new file patch
Chrome ‑ Acceptance | Code patches tests: empty file shows generating content while waiting for patch acceptance
Chrome ‑ Acceptance | Code patches tests: empty file stops showing generating content after canceling patch
Chrome ‑ Acceptance | Code patches tests: loader reset happens when restoring patched executable files
Chrome ‑ Acceptance | Code patches tests: schema editor gets refreshed when code patches are executed on executable files
Chrome ‑ Acceptance | Code patches tests: when code patch is historic (user moved on to the next message), or it was applied, it will render the code (replace portion of the search/replace block) in a standard (non-diff) editor
…

Chrome ‑ Global error: Uncaught Error: Failed to read file at http://test-realm/test/broken.gts: 406  at http://localhost:7357/assets/chunk.253dfbdba2e95e4d19e2.js, line 156670  While executing test: Acceptance | code submode tests > single realm: showing module with a syntax error will display the error 
Chrome ‑ Global error: Uncaught Error: Failed to read file at http://test-realm/test/file2.gts: 404 Not Found at http://localhost:7357/assets/chunk.253dfbdba2e95e4d19e2.js, line 156670  While executing test: Acceptance | Code patches tests: can create new files using the search/replace block 
Chrome ‑ Integration | commands | read-file-for-ai-assistant: rejects reading files not previously attached in the room

♻️ This comment has been updated with latest results.