Update README for styles, vocabulary, and current setup flow

[claude]

Before: The README didn't mention styles, the Enhanced transcripts switch, text shortcuts, or the tap-only / hold-only activation modes. It described setup as a wizard whose window "later hosts settings". Privacy said key terms were the only user setting sent with a request and that Blurt "stores no … transcripts" without mentioning the Developer mode log. Build from source said it "needs only Xcode and Homebrew" and never said how to launch the dev build.

After: The Features list covers Styles and Vocabulary (key terms and text shortcuts), and the trigger bullet mentions the activation modes. Guided setup and Getting started match the current single setup screen, the Style menu, and the Settings window. Privacy now says the selected style's instructions are sent, text shortcuts are expanded locally, and opt-in Developer mode keeps a local dictation log. Build from source lists the prerequisites (Xcode 26+, Homebrew, an Apple Development certificate) and adds open -a "Blurt Dev". The architecture tree's Config/ line now names styles and text shortcuts. Only README.md changes.

What & why

This is a docs-only refresh so the README matches what the app does today. Everything that was already accurate stays as it was: badges, links, the rest of Privacy, and the architecture notes.

How it was tested

Each new claim was checked against the source:

• Styles: Sources/BlurtEngine/Config/StyleProfileStore.swift (profileLimit = 4), App/Blurt/Blurt/Wizard/StyleRow.swift (the "Style" pop-up), and SettingsWindowRoot.swift (the General / Styles / Vocabulary / Advanced panes and the Enhanced transcripts toggle).
• Enhanced transcripts on by default: EnhancedTranscriptsStore.swift.
• Style instructions are sent: AssemblyAITranscriber.swift (customStyle, appended to llm_instruction).
• Text shortcuts are expanded locally and never sent: TextShortcutStore.swift.
• Key terms, text shortcuts, and the Vocabulary pane: KeyTermsStepView.swift, TextShortcutsSection.swift.
• Activation modes: Hotkey/TriggerActivation.swift, HotkeyStepView.swift.
• Microphone picker: MicrophoneStepView.swift.
• Setup screen: WizardView.swift (API key and permissions only).
• Developer mode log: DeveloperModeStore.swift, Pipeline/DictationLog.swift, DeveloperSection.swift.
• Build prerequisites and commands: CONTRIBUTING.md, scripts/bootstrap.sh, scripts/dev-build.sh.

scripts/check.sh --portable passes on Linux, and a standalone markdownlint README.md passes too (the portable run skipped markdownlint because it isn't installed here). The Swift build and tests weren't run locally because this is a docs-only change, so CI on macOS decides the rest.

• scripts/check.sh passes (or CI will, if I'm not on a Mac)
• I read AGENTS.md and this doesn't reintroduce anything deliberately removed
• Docs updated if behavior changed

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