Add Pi Agent as a supported host

## Proposed Feature
This issue proposes adding support for the **Pi Agent** (`@mariozechner/pi-coding-agent`) as a formal host in the GStack framework. Pi is a lightweight, terminal-native AI coding harness that effectively utilizes agentic workflows, making it a natural fit for GStack's multi-persona orchestration.

## Motivation
By adding Pi Agent support, developers who prefer minimal, fast terminal interfaces can leverage GStack's structured team protocols. As Pi Agent (v0.71.1+) gains popularity for its speed and low overhead, formalizing its integration allows for a consistent experience across different host environments.

## Proposed Implementation
Following the `ADDING_A_HOST.md` guidelines, I plan to:
* **Host Configuration**: Create `hosts/pi.ts` defining the `HostConfig`, specifically mapping the global and local skill roots to `~/.pi/agent/skills/gstack`.
* **Host Registry**: Register and export the new config within `hosts/index.ts`.
* **Path Rewrites**: Ensure appropriate rewrites from `.claude/skills` to `.pi/agent/skills` to maintain cross-compatibility.
* **Environment**: Update `.gitignore` to handle `.pi/` specific artifacts.

## Verification Plan
I will validate the addition using the internal framework tools:
1. **Doc Generation**: Run `bun run gen:skill-docs --host pi` to verify skill documentation generation.
2. **Validation**: Run `bun run scripts/host-config-export.ts validate` to ensure the host configuration is correctly exported.
3. **Tests**: Execute existing test suites (`test/gen-skill-docs.test.ts` and `test/host-config.test.ts`) to confirm no regressions.

I have a draft implementation ready and am happy to submit a PR for review.