Add MCP avatar upload sessions

[sametbasbug]

Summary

• add short-lived MCP avatar upload sessions bound to the live grant, exact human account, and target agent
• keep avatar bytes out of MCP JSON/model context and route the browser directly to Orbit
• reuse the existing avatar quarantine, digest, MIME, Images normalization, quota, R2, D1, and media-idempotency pipeline
• add a noindex Orbit upload page with browser-side SHA-256, size/type validation, CSRF, and direct raw upload
• make the upload URL a selector rather than a bearer credential: unauthenticated and wrong-account access is rejected, and revocation/expiry/authority are revalidated on every request
• exclude the private upload handoff page from sitemap output
• remove the nonessential blob-image preview after CodeQL flagged that DOM flow; the final CodeQL gate is clean

Safety and replay

• session TTL: 15 minutes
• PNG/JPEG/WebP only, max 5 MiB
• same session + same bytes safely replays the normalized result
• same session + different bytes conflicts
• bad digest, wrong MIME, oversized payload, expired session, wrong human account, and revoked grant fail closed

Validation

• npm run check: 0 errors / 0 warnings / 2 pre-existing hints
• Slice 5: 28/28 tests passed, including the full new MCP avatar transport chain
• production-live Worker dry-run passed
• production Astro build emits /mcp/avatar-upload/
• sitemap exclusion verified directly
• CodeQL / Analyze / foundation / verify all pass
• git diff --check clean