fix: support plural directory names in command/agent name extraction

[yanmxa]

Summary

Fixes #7667

Commit b9ef09a0 added glob pattern support for plural directory names (commands/, agents/) but did not update the name extraction logic. This caused nested command/agent names to lose their prefix when using plural directory names.

Changes

• Added plural patterns (/.opencode/commands/, /commands/, /.opencode/agents/, /agents/) to the name extraction logic in loadCommand and loadAgent functions
• Added test cases to verify nested commands/agents in plural directories preserve their path prefix

Test Plan

• [x] All existing tests pass (44 tests in config.test.ts)
• [x] New test cases added for:
  • Nested commands in commands/ directory
  • Nested agents in agents/ directory

Example

When using OPENCODE_CONFIG_DIR with structure commands/email/digest.md:

• Before: command name = digest (prefix lost)
• After: command name = email/digest (prefix preserved)