Enhancement: apc install --list should explain required repository skill structure when no skills found

[FZ2000]

Summary

When apc install <repo> --list finds no skills, the error message gives no guidance on the expected repository structure. Users attempting to install skills from arbitrary GitHub repos get no indication of what format is needed.

Steps to Reproduce

apc install FZ2000/apc-cli --list
# Output: No skills found in FZ2000/apc-cli (branch: main).

Actual Behavior

Fetching skill list from FZ2000/apc-cli...
No skills found in FZ2000/apc-cli (branch: main).

No explanation of what structure is expected.

Expected Behavior

Fetching skill list from FZ2000/apc-cli...
No skills found in FZ2000/apc-cli (branch: main).

Skills must be organized as: skills/<name>/SKILL.md
Each SKILL.md should have YAML frontmatter:
  ---
  name: skill-name
  description: What this skill does
  ---

Affected Code

# src/skills.py
# Match: skills/<name>/SKILL.md
parts = path.split("/")
if len(parts) == 3 and parts[0] == "skills" and parts[2] == "SKILL.md":
    names.append(parts[1])

This directory constraint is not communicated anywhere in the CLI or docs.

Environment

• macOS Darwin 25.2.0 (arm64)
• apc version 0.1.0