fix: use harness-specific directory names for skills/agents/commands extraction

[neiii]

Summary

• Fix OpenCode skills/agents/commands not being detected due to hardcoded plural directory names
• Add fallback support to check alternate directory form (singular ↔ plural)

Problem

Bridle was hardcoding plural directory names (skills/, agents/, commands/) when extracting resources from profiles. However, OpenCode uses singular directory names (skill/, agent/, command/), causing all OpenCode skills, agents, and commands to go undetected.

# What Bridle was looking for:
~/.config/bridle/profiles/opencode/test/skills/    ❌ doesn't exist

# What OpenCode actually uses:
~/.config/bridle/profiles/opencode/test/skill/     ✓ exists

Solution

1. Use harness-provided paths: Extract the actual directory name from harness_locate's DirectoryResource.path instead of hardcoding strings

2. Add fallback support: If the primary directory is empty/missing, check the alternate form:

   • OpenCode: tries skill/ first, falls back to skills/
   • Claude: tries skills/ first, falls back to skill/

Changes

• Added dir_name_from_path() helper to extract directory name from path
• Added fallback_dir_name() to get alternate singular/plural form
• Modified extract_skills(), extract_agents(), extract_commands() to use dynamic paths with fallback

Testing

• Added unit tests for helper functions
• Added test verifying OpenCode uses singular directory names
• Verified with cargo run -- profile show opencode test - now correctly detects skills, agents, and commands