feat(harness): add EffectTS tool layer starting with pluggable bash tool - #6

chore(harness): bump effect 4.0.0-beta.86

Implement the agent tool layer

- Tool.define/make/implement + Effect Schema as the source of truth;
  toAikitTool derives a provider-safe JSON Schema for the aikit wire
- Executor pipeline: decode -> run (scoped, interruptible) -> encode into
  aikit's content/details; failureMode "return" (tool error result) vs
  "error" (propagates to the error channel); aborted on interrupt; defects
  on undeclared failures; fails fast on duplicate tool names
- ToolShell capability with swappable backends: local (real OS shell on
  Effect's ChildProcessSpawner, with an explicit SIGTERM->SIGKILL escalation
  finalizer so timeout/interrupt is a hard bound) and fromSandboxShell
  (just-bash / remote). forceKillAfter is per-call or adapter default
- ToolProgress service (noop layer); truncate utils ported from pi
- bash tool (buffered variant A) — backend swapped via layer, no tool change

Vercel exec now runs detached and kills the remote process on interrupt/timeout;
adds optional ISandboxExe.stream (Vercel via Command.logs) bridged to ToolShell.stream.