fix(tests): redirect implementation tests to workflow files after extraction

After extracting debug.md and thread.md implementations to workflow files
and renaming extract_learnings.md, existing tests still referenced the
old locations:

- debug-session-management.test.cjs: commands/gsd/debug.md → workflows/debug.md
- thread-session-management.test.cjs: commands/gsd/thread.md → workflows/thread.md
- extract-learnings.test.cjs: extract_learnings.md → extract-learnings.md
- enh-2430-learnings-consumption.test.cjs: extract_learnings.md → extract-learnings.md

Also adds <available_agent_types> block and TEXT_MODE fallback note to
get-shit-done/workflows/debug.md to satisfy the spawn-type-consistency
(#1357) and AskUserQuestion text-mode fallback (#2012) contract tests
that scan all workflow files.

tests/thread-session-management.test.cjs

@@ -11,7 +11,7 @@ const path = require('path');
 
 describe('thread session management (#2156)', () => {
   const threadCmd = fs.readFileSync(
-    path.join(__dirname, '..', 'commands', 'gsd', 'thread.md'),
+    path.join(__dirname, '..', 'get-shit-done', 'workflows', 'thread.md'),
     'utf8'
   );