Add composer completions and update OpenCode V2 contract

[eivindjonassen]

Summary

• Add caret-aware slash command completion and file, skill, and agent mentions to the mobile composer.
• Persist structured mention ranges in encrypted drafts and preserve safe command recovery across restarts.
• Upgrade the generated OpenCode client and notification plugin contract to beta 18387, including command, event, error, and interrupt changes.
• Extend deterministic fixtures, compatibility records, migrations, and tests for the new behavior.

Verification

• pnpm check
• 292 mobile tests
• 61 adapter tests
• iOS and Android Hermes exports
• pnpm native:doctor with 18/18 checks
• iOS and Android EAS preview builds submitted