fix(web): the Deploy card explains a rejected package (CL-8579)

[TheGreatAxios]

Summary

• A Deploy package attempt with a wrong or missing field used to vanish silently — the person just saw two code blocks and no button, with no explanation.
• deployablePackage now distinguishes a valid package, a named rejection, and "not a package attempt at all," so the person always sees why a Deploy card didn't appear.

Changes

• deployablePackage / deployablePackageFromBody / resolveMessagePackage return a PackageOutcome: a parsed package, { reason } naming the missing or invalid field in plain words, or null when neither contract file is present.
• name now comes from definition.json, falling back to package.json's name when definition.json omits it; package.json is parsed with arktype too, and unknown fields anywhere are ignored rather than fatal.
• message-attachments.tsx renders a small rejection card in the Deploy card's visual family, naming the reason, with no Deploy button.

Testing

• bun test apps/web/src/chat/deployable-package.test.ts
• bun run check (typecheck, lint, fmt, unit tests) from the repo root

[linear-code]

CL-8579