refactor(skill-loader): delete dead parallel/worker scanner chain (#61)

Removes 1077 LOC of unreachable code across three files in
src/features/opencode-skill-loader/:

- blocking.ts + blocking.test.ts - discoverAllSkillsBlocking had
  zero production callers (only its own test).
- discover-worker.ts - worker entrypoint used only by blocking.ts.
- async-loader.ts + async-loader.test.ts - discoverSkillsInDirAsync,
  loadSkillFromPathAsync, loadMcpJsonFromDirAsync, and
  mapWithConcurrency had zero production callers after blocking.ts
  became dead.

All production code paths (loader.ts wrappers + config-source-discovery.ts)
route through skill-directory-loader.ts::loadSkillsFromDir, which is
preserved unchanged.

Verification:
- bun run typecheck: clean
- bun test: 5602 pass, 0 fail
  (5612 -> 5602; the 10 deleted tests covered deleted code only)
- lsp_diagnostics on src/: 0 errors
- No dangling references: grep for deleted symbols and filenames
  returns no matches in src/ outside the deleted files