feat(pstack): add Agent Plugins v1 compatibility

[mantrakp04]

Summary

• add a root plugin.json targeting Agent Plugins v1.0.0
• make the poteto-mode skill identifier match its directory and preserve the display label in standard metadata
• document the portable boundary: skills/ travels across Agent Plugins clients, while the two subagents remain Cursor-specific

Context

pstack already uses the standard fixed skills/<name>/SKILL.md layout, but Agent Plugins clients had no root manifest to discover it. Its main entry point also declared name: Poteto Mode, which violates the shared Agent Skills identifier rules.

The existing .cursor-plugin/plugin.json stays in place, so the Cursor marketplace keeps its native metadata and agents/ support. The identifier change overlaps #240 and #241; it is included here because portable pstack needs that entry point to be valid on its own.

Validation

• node scripts/validate-plugins.mjs
• validated pstack/plugin.json with AJV 2020 against the canonical v1.0.0 schema
• ran skills-ref validate across all 44 skills; after this identifier fix, its remaining diagnostics are only the existing Cursor-specific frontmatter extensions
• git diff --check HEAD^ HEAD

Closes #237

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Note

Low Risk
Metadata and documentation only; no runtime or security-sensitive logic changes.

Overview
Adds a root plugin.json so Agent Plugins v1 clients can discover and install pstack/ without relying on Cursor-only packaging. The manifest carries version, description, and keywords; .cursor-plugin/plugin.json is unchanged for the Cursor marketplace.

poteto-mode skill frontmatter now uses the directory-aligned identifier poteto-mode instead of Poteto Mode, with metadata.cursor-display-name preserving the human label in Cursor.

README install section documents installing via any Agent Plugins client, and clarifies portability: skills/ is portable; agents/ subagents stay Cursor-specific.

^{Reviewed by Cursor Bugbot for commit 501f5cb. Configure here.}