feat: add .agents/skills as a skill provider directory

[Br1an67]

TLDR

Adds .agents (with an "s") as a skill provider directory, following the emerging cross-tool convention (agentskills/agentskills#15). One-line change to SKILL_PROVIDER_CONFIG_DIRS with updated tests.

Dive Deeper

Currently only .qwen/skills and .agent/skills are recognized. Multiple AI agent tools are converging on .agents/skills as a standard directory (Cursor, Claude Code, etc.). This adds it as a third option.

Change: packages/core/src/config/storage.ts line 15:

- export const SKILL_PROVIDER_CONFIG_DIRS = ['.'qwen\, '.'agent'];
+ export const SKILL_PROVIDER_CONFIG_DIRS = ['.'qwen\, '.'agent', '.'agents'];

Precedence order: .qwen > .agent > .agents (first match wins in deduplication). All downstream code (skill-manager.ts, storage.ts getUserSkillsDirs()) automatically picks up the new directory via the shared constant.

Reviewer Test Plan

1. Create .agents/skills/test-skill/SKILL.md in project root
2. Run qwen and verify the skill appears in /skills listing
3. Verify .qwen and .agent skills still take precedence over .agents duplicates

Testing Matrix

	🍏	🪟	🐧
npm run	❓	❓	❓

Linked issues / bugs

Closes #2155

[tanzhenxin]

@Br1an67 Pls help resolve the unit tests errors, we would love to merge this PR once they are resolved.

[Br1an67]

Hi @tanzhenxin, thanks for the heads up! I've rebased onto the latest main and force-pushed. All 40 tests in skill-manager.test.ts pass locally — the CI failure was likely due to a stale merge base. The new CI run should be green now.

[tanzhenxin]

@Br1an67 Thanks for your quick response! But I think maybe we should follow @karlhorky 's advise, I mean the standard name is ".agents", not ".agent". Pls check.

[Br1an67]

Done! Updated to replace .agent with .agents per @karlhorky's suggestion and your feedback.

SKILL_PROVIDER_CONFIG_DIRS is now ['.qwen', '.agents']. Tests updated accordingly — all 40 pass locally. CI should be green shortly.

[tanzhenxin]

LGTM!