feat: add support for Pi (pi.dev) coding agent (Fission-AI#735)

* feat: add support for Pi (pi.dev) coding agent

Add Pi as a supported tool in OpenSpec with full adapter implementation.

Changes:
- Create pi.ts adapter for command generation
- Register adapter in registry and export from index
- Add Pi to AI_TOOLS config with .pi skills directory
- Add tests for piAdapter following existing patterns
- Update supported-tools.md documentation

Pi uses:
- Skills: .pi/skills/ (Agent Skills standard)
- Prompts: .pi/prompts/*.md (with description frontmatter)

Closes Fission-AI#732

* fix: add Pi to LEGACY_SLASH_COMMAND_PATHS for test compliance

* style: add trailing newline to pi.ts

* fix: correct legacy cleanup pattern for Pi (opsx-*.md not openspec-*.md)

* fix: add YAML escaping for Pi adapter to handle special characters in descriptions

- Add escapeYamlValue() function to properly escape YAML special characters
- Apply escaping to description field in frontmatter
- Add tests for YAML special character escaping (colons, quotes, newlines)

This follows the same pattern used by cursor, claude, and windsurf adapters.

* fix: remove Pi from LEGACY_SLASH_COMMAND_PATHS

Pi was never supported in pre-1.0 versions, so no legacy cleanup is needed.
Per reviewer feedback: this is only for tools from pre-1.0 OpenSpec.

* test: relax legacy-cleanup registry coverage invariant

---------

Co-authored-by: Tabish Bidiwale <30385142+TabishB@users.noreply.github.com>
Co-authored-by: TabishB <tabishbidiwale@gmail.com>

Author: carlosapgomes

docs/supported-tools.md

@@ -31,6 +31,7 @@ For each tool you select, OpenSpec installs:
 | Kilo Code | `.kilocode/skills/` | `.kilocode/workflows/` |
 | Kiro | `.kiro/skills/` | `.kiro/prompts/` |
 | OpenCode | `.opencode/skills/` | `.opencode/command/` |
+| Pi | `.pi/skills/` | `.pi/prompts/` |
 | Qoder | `.qoder/skills/` | `.qoder/commands/opsx/` |
 | Qwen Code | `.qwen/skills/` | `.qwen/commands/` |
 | RooCode | `.roo/skills/` | `.roo/commands/` |
@@ -56,7 +57,7 @@ openspec init --tools all
 openspec init --tools none
 ```
 
-**Available tool IDs:** `amazon-q`, `antigravity`, `auggie`, `claude`, `cline`, `codebuddy`, `codex`, `continue`, `costrict`, `crush`, `cursor`, `factory`, `gemini`, `github-copilot`, `iflow`, `kilocode`, `kiro`, `opencode`, `qoder`, `qwen`, `roocode`, `trae`, `windsurf`
+**Available tool IDs:** `amazon-q`, `antigravity`, `auggie`, `claude`, `cline`, `codebuddy`, `codex`, `continue`, `costrict`, `crush`, `cursor`, `factory`, `gemini`, `github-copilot`, `iflow`, `kilocode`, `kiro`, `opencode`, `pi`, `qoder`, `qwen`, `roocode`, `trae`, `windsurf`
 
 ## What Gets Installed