feat(web): add bounded attachment admission rules

[seekskyworld]

Problem

Implements the admission-contract slice of #344. Web attachments had no shared, testable bounds for count, filename, MIME, per-file size, or aggregate size.

Value

Future file/image upload paths can fail closed before touching workspace or Session authority.

Approach

Add a pure protocol validator with explicit supported MIME types, filename traversal protection, count and UTF-8-independent byte limits. No upload storage or prompt behavior is changed.

Validation

• node --test --experimental-strip-types tests/web/attachments.test.ts
• npx tsc --noEmit
• git diff --check

Impact

• User-visible behavior: establishes bounded attachment admission errors for future UI.
• Model-visible context/tools: none.
• Runtime/lifecycle: none.
• Persisted config/data: none.
• Compatibility/risk: additive protocol utility; upload flow remains unimplemented.