feat(hosts): add Pi as a new host target

[mlarabi]

Summary

Add Pi as a new supported host. Pi is an open-source AI coding agent harness that uses Claude Opus under the hood, with its own skill discovery system following the Agent Skills specification.

New host config (hosts/pi.ts):

• Paths matching Pi's skill discovery (~/.pi/agent/skills/ global, .pi/skills/ project-local)
• Agent Skills spec compliance: 1024 char description limit, allowlist frontmatter mode
• frontmatter.prefixName: true — Pi validates that name: matches the parent directory name (review → gstack-review)
• Lowercase tool name rewrites (Bash → bash, Read → read, AskUserQuestion → ask_user_question)
• Boundary instructions include ~/.pi/ so Codex avoids reading Pi skill files
• Self-identity rewrites where skills refer to "this Claude Code window" → "this Pi window"
• PR footer removed (Pi doesn't inject unsolicited branding)
• Full learnings mode (same model capabilities as Claude)
• Co-author trailer: Claude Opus 4.6 via Pi

New config option (scripts/host-config.ts):

• frontmatter.prefixName — when true, the name: field in generated SKILL.md is prefixed with gstack- to match the output directory name. Pi requires this at runtime; other hosts don't enforce it.

Setup script (setup):

• --host pi accepted, pi added to auto-detect (command -v pi)
• Step 1d: generates .pi/ skill docs via bun run gen:skill-docs --host pi
• Step 6c: creates ~/.pi/agent/skills/gstack/ runtime root with symlinked assets (bin/, browse/dist, browse/bin, ETHOS.md, review sidecars), copies generated Pi-native SKILL.md files into ~/.pi/agent/skills/gstack-*/

Upgrade template (gstack-upgrade/SKILL.md.tmpl):

• Detects .pi/skills/gstack (local-git) and ~/.pi/agent/skills/gstack (vendored-global) install paths
• Vendored copy sync (Step 4.5) checks .pi/skills/gstack alongside .claude/skills/gstack

Uninstall (bin/gstack-uninstall):

• Removes ~/.pi/agent/skills/gstack* (global Pi install)
• Removes .pi/skills/gstack* (project-local Pi sidecar)
• Does NOT rmdir .pi/ itself — Pi owns that directory and may have settings.json, sessions, or other state there

Infrastructure:

• Registered in hosts/index.ts (9 hosts total)
• .pi/ added to .gitignore
• Host count assertion updated in tests (8 → 9)
• Setup validation tests updated for Pi host detection
• Golden baselines regenerated
• Pi added to README agents table

Follows docs/ADDING_A_HOST.md step-by-step. Zero changes to the generator's core logic — only a 3-line prefixName implementation in transformFrontmatter.

Design decisions

Always prefixed — no short names for Pi

Pi validates at runtime that the name: field in SKILL.md matches the parent directory name. The generator outputs skills to gstack-* directories (e.g., gstack-review/SKILL.md), so the frontmatter must say name: gstack-review. Short names (/skill:review in a directory called review/) would require both renaming the install directory and rewriting the frontmatter during setup — fragile and not worth the complexity.

The setup script skips the prefix prompt entirely for --host pi. The check uses $HOST (available early, before INSTALL_* flags are resolved) rather than $INSTALL_PI (set later) to avoid the prompt showing before the flag exists.

Suppress gstack-relink warning for Pi

When skill_prefix: true is saved in ~/.gstack/config.yaml, gen:skill-docs prints a reminder to run gstack-relink to re-apply Claude symlink name patches. This is irrelevant for Pi (Pi doesn't use symlinks — it copies SKILL.md files). The warning is suppressed when --host pi is the active generation target.

Don't rmdir .pi/ on uninstall

Other host directories (.agents/, .factory/) are created exclusively by gstack and can be safely removed. .pi/ is owned by Pi itself and may contain settings.json, session data, or other user config. Uninstall removes gstack* entries inside .pi/skills/ but leaves the directory intact.

Boundary instructions include ~/.pi/

When Codex is invoked as a second opinion from a Pi session, the boundary instruction now tells Codex to avoid ~/.pi/ alongside ~/.claude/ and ~/.agents/. Without this, Codex could waste tokens reading Pi skill definitions.

Test Coverage

All new code paths covered by existing parameterized smoke tests that auto-pick up new hosts.

bun run scripts/host-config-export.ts validate  → All 9 configs valid
bun run skill:check                             → Pi: 35/35 skills, freshness ✅
bun test test/host-config.test.ts               → All pass (host count, uniqueness, validation)
bun test test/gen-skill-docs.test.ts            → All pass (freshness, setup validation, Pi host detection)
grep -r ".claude/skills" .pi/skills/            → No leakage

Pre-Landing Review

No issues found. One new config file (hosts/pi.ts), one new config option (prefixName), setup script Pi section, upgrade template awareness, uninstall support, and standard test/gitignore updates. No changes to existing host configs or generator core logic.

Test plan

• bun run gen:skill-docs --host pi generates 35 skills
• Pi SKILL.md name: fields match directory names (gstack-review, gstack-ship, etc.)
• bun run scripts/host-config-export.ts validate — all 9 configs valid
• bun run skill:check — Pi: 35/35 skills, 0 missing, freshness ✅
• bun test test/host-config.test.ts — all pass
• bun test test/gen-skill-docs.test.ts — all pass (including setup Pi validation)
• grep -r ".claude/skills" .pi/skills/ — empty (no path leakage)
• bun run build — full build succeeds
• ./setup --host pi — no prefix prompt, installs to ~/.pi/agent/skills/, Pi discovers all 35 skills
• ./bin/gstack-uninstall — lists Pi paths in confirmation, removes gstack* entries, preserves .pi/