Full iOS 27 Siri and Apple Intelligence integration, once 27 ships

[jjalangtry]

Goal: full iOS 27 Siri and Apple Intelligence integration. This is a wanted
direction, not just a revert.

What was removed, and why

Mail intents and entities were bound to Apple Intelligence assistant schemas via
@AppIntent(schema:), @AppEntity(schema:) and @AppEnum(schema:). That binding
was removed in #167 because the schema shape differs between SDKs:

Construct	iOS 26 SDK	iOS 27 SDK
AssistantSchemas.Enum.mail (.mail.category)	absent	present
.mail.openDraft	absent	present
.mail.openMessage	absent	present
.mail.message	no schema-bound category	requires category of the schema enum type

The app must build against the released iOS 26 SDK because App Store Connect
rejects beta-SDK uploads with error 90534 (see #165), so the iOS 27 schema
code could not compile. Both Xcode Cloud workflows are pinned to Xcode 26.6 and
the deployment target is 26.0 (#166).

Everything still exists as plain App Intents — same types, queries, display
representations and perform bodies. Siri and Shortcuts are unaffected. Only
the assistant-schema binding is gone.

When iOS 27 reaches RC or release

1. Repoint both Xcode Cloud workflows to the iOS 27 Xcode (the API exposes
   xcodeVersion on ciWorkflows; environment variables it does not).
2. Raise the deployment target in apps/ios/project.yml.
3. Restore the schema bindings in MailAppSchemaIntents.swift and
   MailAppSchemaEntities.swift — git history at 05467a6^ has the exact
   prior form, including the two OpenIntents and the category enum.

Then go further

The mail domain was the only one bound. Worth surveying the full iOS 27
assistant schema catalogue for the domains Albatross actually covers — mail,
calendar, tasks, files, and the brief — rather than restoring mail alone.