Add Pi `/plan` Mode

Summary

Added a project-local Pi extension that lets users type /plan to activate a guarded planning mode, then /plan off or /implement to return to implementation mode.

Changes Made

Created .pi/extensions/plan-mode.ts.
Registered a /plan command that enables plan mode.
Registered a /implement command and /plan off argument to disable plan mode.
Added tool-call guards that block write, edit, and common mutating shell commands while plan mode is active.
Added a turn document for this change.

Context

Pi does not ship with built-in plan mode. Its documented extension system supports custom slash commands and tool-call interception, which fits this workflow without patching Pi internals.

Important Implementation Details

/plan          # enable planning guardrails
/plan off      # disable planning guardrails
/implement     # disable planning guardrails

When active, plan mode appends explicit system instructions before each agent turn and blocks file mutation tools. Bash commands are screened with conservative patterns for filesystem, git, package-manager, and Docker mutations.

Validation

Ran NODE_PATH=/opt/homebrew/lib/node_modules bun --check .pi/extensions/plan-mode.ts successfully.
Initial bun --check .pi/extensions/plan-mode.ts failed because the Pi package is installed globally, not as a repo dependency. Retried with NODE_PATH pointed at Homebrew global Node modules.

Issues, Limitations, and Mitigations

The bash mutation detector is intentionally conservative but cannot perfectly classify every shell command. Direct Pi write and edit calls are fully blocked.
The extension is project-local, so it activates automatically for Pi sessions launched in this repository. To use it everywhere, copy it to ~/.pi/agent/extensions/.

Follow-up Work

No required follow-up work. Beads issue: islandflow-hio.

Add Pi /plan Mode