refactor: adopt Pi harness built-ins

[timbrinded]

Summary

• adopt Pi 0.80.10 session, model-runtime, tool, completion, widget, and selector built-ins
• replace redundant workflow harness and custom run-navigation infrastructure with Pi-native surfaces
• preserve live host provider configuration and selected-model credentials in workflow child sessions
• add a bounded Dynamax shine cue with static/off preferences while preserving custom-editor ownership
• tighten durable-run completion scope, workflow viewer layout, and runtime module boundaries
• refocus the README on user-facing workflows and installation

Changes

• use createAgentSessionServices and createAgentSessionFromServices for production subagent sessions
• isolate child-runtime provider synchronization, including shared-cwd removal and runtime/stored API-key handling
• delete the custom workflow run navigator and use native run/action selectors in TUI and RPC modes
• derive workflow-run parsing and completions from one typed action list and keep retained-run lookup scoped to the active session cwd
• use run-scoped string widgets and shared ANSI-aware viewer row/viewport helpers
• split Dynamax terminal rendering and animation from activation/session lifecycle code
• preserve an existing custom editor when decoration fails, cancel stale animation work, and avoid overwriting later editor replacements
• adopt Pi-native command completions, getAgentDir(), built-in tool provenance and noTools, getLastAssistantText(), and RPC-safe UI behavior
• enforce Pi 0.80.10 as the minimum runtime

User impact

Workflow commands now use Pi-native selection and completion behavior across supported UI modes. Durable-run completions stay attached to the active project session. Dynamax performs one short shine sweep and then remains statically highlighted; PI_DYNAMAX_EFFECT=static or off reduces or disables the cue. Existing custom editors remain authoritative when they cannot be decorated.

Validation

• bun run typecheck
• bun run test: 479 passed, 0 failed across 57 files
• git diff --check
• index.ts: 943 lines
• PR diff: 2,728 additions, 1,634 deletions across 54 files

This PR does not bump or publish a release.