fix: support both .claude/skills/ and .agents/skills/ for skill discovery (#759)

* fix: use `.claude/skills/` instead of `.agents/skills/` in agent and workflow skill references

Claude Code resolves project skills from `.claude/skills/` (project-level)
and `~/.claude/skills/` (user-level). The `.agents/skills/` path is the
universal/IDE-agnostic convention that Claude Code does not resolve, causing
project skills to be silently ignored by all affected agents and workflows.

Fixes #758

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: support both `.claude/skills/` and `.agents/skills/` for cross-IDE compatibility

Instead of replacing `.agents/skills/` with `.claude/skills/`, reference both
paths so GSD works with Claude Code (`.claude/skills/`) and other IDE agents
like OpenCode (`.agents/skills/`).

Addresses review feedback from begna112 on #758.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Stephen Miller <Stephen@betterbox.pw>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

get-shit-done/workflows/execute-phase.md

@@ -122,7 +122,7 @@ Execute each wave in sequence. Within a wave: parallel if `PARALLELIZATION=true`
        - .planning/STATE.md (State)
        - .planning/config.json (Config, if exists)
        - ./CLAUDE.md (Project instructions, if exists — follow project-specific guidelines and coding conventions)
-       - .agents/skills/ (Project skills, if exists — list skills, read SKILL.md for each, follow relevant rules during implementation)
+       - .claude/skills/ or .agents/skills/ (Project skills, if either exists — list skills, read SKILL.md for each, follow relevant rules during implementation)
        </files_to_read>
 
        <success_criteria>