Add Pi provider integration

[isthatyousaf]

Summary

Adds Pi as a first-class DP Code provider using the direct TypeScript SDK from @mariozechner/pi-coding-agent.

• Adds Pi contract/model support, including ProviderKind, model selection, provider start options, provider discovery, runtime raw source, static fallback model, thinking levels, and display metadata.
• Implements a Pi provider adapter with SDK-backed session start/resume, prompt dispatch, streaming event mapping, interruption, stop/dispose, thread reads, compaction, runtime model discovery, context usage tracking, native tool lifecycle mapping, native commands, and native skills.
• Registers Pi across server provider layers, session persistence, provider health, status ordering, and provider service routing.
• Wires Pi into the web app provider/model picker, runtime model search and favourites, settings, Pi logo, composer traits, slash commands, skill mentions, context-window display, draft persistence, thread activation, and handoff flows.
• Ensures Pi runs as full-access/default mode only, with no approval-required bridge and no synthetic plan mode.
• Supports handoff to and from Pi, including Pi target runtime/interaction coercion and context-window handling so target-provider context limits win after handoff.

Notable behavior

• Pi sessions persist through the Pi SDK session JSONL path stored in resumeCursor.sessionFile.
• Runtime Pi model discovery is authoritative when available; openai/gpt-5 remains the static fallback.
• Pi thinking effort maps to the SDK thinkingLevel option and is preserved in composer drafts and runtime model selections.
• Pi native tools render through canonical DP Code runtime items, with read/list/find/grep/write/edit/bash mapped to existing tool item affordances.
• Pi native slash commands and skills are discoverable in the composer. Pi excludes DP Code plan/default mode commands because Pi is always default-mode in this integration.

Verification

• bun run --cwd packages/shared test src/model.test.ts — passed, 55 tests
• bun run --cwd apps/server test src/provider/Layers/PiAdapter.test.ts src/provider/Layers/ProviderHealth.test.ts — passed, 49 tests
• bun run --cwd apps/web test src/composerDraftStore.test.ts src/hooks/useComposerCommandMenuItems.test.ts src/lib/threadHandoff.test.ts src/providerModelOptions.test.ts src/components/chat/composerProviderRegistry.test.tsx — passed, 101 tests
• bun run typecheck — passed across 7 packages
• bun run fmt — passed
• bun run lint — passed; repo still reports existing unrelated warnings, and touched Pi paths were checked separately with no new warnings

Suggested review focus

• Pi adapter lifecycle: session start/resume, prompt dispatch, interruption, and cleanup.
• Runtime model discovery and thinking-level behavior for SDK models.
• Native Pi tool, command, and skill event/UX mapping.
• Handoff behavior between Pi and Codex/Claude/Gemini/OpenCode, especially full-access/default coercion and context-window display.
• Provider health/auth messaging for installed Pi CLI with or without configured provider credentials.

Deployment notes

• Adds @mariozechner/pi-coding-agent to the server package.
• Adds Pi-related app settings keys and provider metadata.
• No database migration or external infrastructure change is required.