init-agents: add a tool-agnostic `.agents/skills/` flavour (--loop=agents)

[kwojcik-blockether]

Summary

spel init-agents can only scaffold into two tool-specific layouts — --loop=opencode (.opencode/agents/, .opencode/skills/) and --loop=claude (.claude/agents/, .claude/skills/). It has no target for the tool-agnostic .agents/skills/<name>/ convention that a growing number of repos/harnesses now load skills from (a single .agents/skills/ tree, SKILL.md + references/, agent nested under the skill).

Because of this gap, adopting spel in an .agents/-based repo requires a manual dance: generate with --loop=claude, then hand-move .claude/skills/spel → .agents/skills/spel, relocate .claude/agents/spel.md under the skill, fix the compatibility: frontmatter, and delete the empty .claude dirs. That's exactly what I just had to do by hand.

Request

Add a general/agnostic flavour, e.g. --loop=agents (or a --dir .agents/skills override), that scaffolds directly into:

.agents/skills/spel/
  SKILL.md                 # frontmatter: name / description / version (compatibility: agents)
  references/*.md
  agents/spel.md           # agent nested inside the skill dir

Why

• .agents/skills/ is emerging as the cross-tool ("general flavour") place to drop skills so one copy serves multiple agent runtimes instead of duplicating per-vendor dirs.
• Keeps init-agents a one-shot for those repos (no post-move cleanup).
• Small change: it's the same generated content, just a different destination + compatibility value.

Current workaround

spel init-agents --ns <ns> --loop=claude --no-tests
mv .claude/skills/spel .agents/skills/spel
mkdir -p .agents/skills/spel/agents && mv .claude/agents/spel.md .agents/skills/spel/agents/spel.md
# edit SKILL.md: compatibility: opencode -> agents
rm -rf .claude

Tested with spel 0.9.6.