fix: ignore legacy skill dirs in agent discovery

[affsantos]

Summary

Legacy .agents/**/*.md discovery currently treats Pi skill files under .agents/skills/** as executable subagents when they contain name and description frontmatter. That makes valid skill packages like .agents/skills/foo/SKILL.md show up in subagent list as runnable agents.

This keeps the legacy .agents compatibility behavior, but excludes .agents/skills/** from agent discovery. The intent is to preserve custom legacy agents under .agents/ while respecting Pi's documented skill location under .agents/skills/.

Why

Pi documents both canonical and legacy skill locations, including:

• ~/.pi/agent/skills/
• ~/.agents/skills/
• .pi/skills/
• .agents/skills/

Since skills also use markdown/frontmatter, broad .agents/**/*.md agent discovery can accidentally classify skill docs as executable agents. This is confusing in subagent list, where skill instruction packages can appear to be runnable subagents.

Changes

• Add an optional directory-skip predicate to recursive markdown discovery.
• Skip skills subtrees only when scanning a legacy .agents root for agents.
• Add coverage that .agents/skills/*.md and .agents/skills/*/SKILL.md are ignored while ordinary legacy .agents/*.md agents still load.

Validation

• node --experimental-strip-types --test test/unit/agent-frontmatter.test.ts

Note: npm test -- test/unit/agent-frontmatter.test.ts starts the full unit suite in this checkout and hits unrelated missing peer dependencies for TUI-related tests (@earendil-works/pi-tui, @earendil-works/pi-coding-agent). The targeted test file passes with Node's strip-types runner.