Fix dictation prompt role separation

[sdhilip200]

Description

Fixes dictation post-processing prompt composition so cleanup instructions are sent as the system message and the transcript is sent separately as tagged user content. Legacy custom prompts that still use ${transcript} continue to render with replacement behavior.

Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📝 Documentation update

Related Issues

• Closes [🐞 BUG] AI Request formed in a way that is much more likely to lead to failed instruction following (answering the transcript instead of cleaning it). #388

Testing

• Tested on Intel Mac
• Tested on Apple Silicon Mac
• Tested on macOS 26.2
• Ran linter locally: swiftlint --strict --config .swiftlint.yml Sources
• Ran formatter locally: swiftformat --config .swiftformat Sources
• Built locally: sh build_incremental.sh

Additional validation run locally:

• xcodebuild clean build -project Fluid.xcodeproj -scheme Fluid -destination platform=macOS,arch=arm64 -derivedDataPath /private/tmp/FluidVoiceDerivedData CODE_SIGN_IDENTITY="" CODE_SIGNING_REQUIRED=NO CODE_SIGNING_ALLOWED=NO -quiet
• xcodebuild test -project Fluid.xcodeproj -scheme Fluid -destination platform=macOS,arch=arm64 -derivedDataPath /private/tmp/FluidVoiceDerivedData CODE_SIGN_IDENTITY="" CODE_SIGNING_REQUIRED=NO CODE_SIGNING_ALLOWED=NO -quiet
• git diff --check

Notes

• The SwiftPM lockfile updates modelcontextprotocol/swift-sdk to 0.12.1 because the previously pinned 0.10.2 fails to compile under Xcode 26.5 / Swift 6 strict-concurrency checking.
• sh build_incremental.sh currently fails in this checkout because it references build_dev.sh, which is not present in the repository. Direct xcodebuild build and test commands passed instead.
• Full swiftformat --config .swiftformat Sources was not committed because it rewrites broad pre-existing formatting drift outside this focused bugfix.

Screenshots / Video

N/A - no UI changes.