Pi 0.84.0: Fullscreen TUI, Mermaid/LaTeX rendering, per-directory context overrides, extensive API breaks

New Features

• Fullscreen TUI mode: runtime switching between regular/fullscreen, sticky editor and footer, independently scrollable transcript, draggable scrollbars
• Mermaid diagram rendering in interactive transcripts
• LaTeX Unicode math rendering in interactive transcripts
• Per-directory context overrides via AGENTS.override.md
• Advanced custom model sampling: arbitrary OpenAI-compatible samplingParams, opt-in vLLM thinking_token_budget
• Built-in Baseten provider with authentication and model support

Breaking Changes

• ModelsStreamTransforms renamed to ModelsRequestTransforms (header transformation now applies to all authenticated provider requests)
• JSON/RPC message_update events now emit only assistantMessageEvent deltas; callers must assemble partial messages between message_start and message_end
• ModelRegistry.getApiKeyAndHeaders() returns ProviderHeaders with string | null values preserving null header-deletion markers
• ModelRegistry.refresh() accepts ModelsRefreshOptions and returns ModelsRefreshResult
• ModelRuntime.setRuntimeApiKey() accepts auth cancellation options instead of catalog refresh options
• Provider refresh context: replaced dynamic store access with read-only context.stored snapshot and generation-checked context.publish() transaction
• v4 lane-based Session, SessionStorage, and SessionRepo APIs replacing the legacy harness session model
• v2 session/AgentHarness API promoted from experimental to default export; experimental subpaths removed
• Legacy JSONL and in-memory repository APIs removed