Add pi agent support

[kleb]

Add pi agent support

Adds pi as a supported agent backend alongside claude, codex, copilot, and grok.

Changes

• Added "pi" to agent validation set
• Added pi case to agent launch command
• Updated README prerequisites list

Testing

Production tested with four-pack workflow on C++ project.

Usage

window coder pi coder
window architect pi architect batch --model claude-opus-4 --thinking high

[RicJMC]

Tested this PR locally (two-pack on a small Python toy repo, pi 0.84.4 as backend for both roles, a flash-tier model via --model). Full cycle closed without intervention after the initial task: coder TDD commit → git_handoff (priority 50) → cleaner (batch) merge + hardening → reverse handoff (priority 00) → coder merge → tests green. ~$0.02 and ~4 minutes for the closed cycle.

Three findings from our first (failed, documented) run, all small:

1. ./swarm silently re-downloads swarmforge/scripts/ from main when scripts/shared-articles/ is missing — this clobbered the local "pi" patch on first boot ("Unsupported agent 'pi'"). Workaround: create scripts/shared-articles/ before first boot. Might be worth warning instead of silently overwriting modified scripts.

2. handoffd.bb notify! sends the wake text + C-m + C-j. In the pi TUI, C-m does not submit the input (4/4 controlled failures); tmux send-keys Enter does (2/2). One-line patch: C-m → Enter in notify!.

3. Conf order is semantic, not cosmetic: require-master-worktree! needs exactly one master worktree window (so coder pi coder will not boot), and the last window is the "role terminal" its inbound git_handoffs are marked non-forwarding, and the helper refuses any git_handoff from whoever is processing one. With cleaner, coder order, the cleaner's return handoff (00) is refused and its commit is orphaned on its branch. The two-pack default order (coder first, cleaner last) is load-bearing worth a line in the README.

Report from a scripted two-run experiment, agent-assisted;
Happy to share the patch diffs if useful.