docs: address review feedback on ADR-024 dynaconf configuration management

Address all 9 review points from PR #680:
- Fix post_hooks code examples to use correct return-dict signature
- Expand test isolation section with fixture factory, DynaconfDict, and
  reload patterns
- Document ignore_unknown_envvars silent failure mode in Negative
  Consequences and add env var audit to Phase 1 checklist
- Fix NEXTCLOUD_HOST validator to be unconditional (required in all modes)
- Document environments=True edge cases (unset mode, ENV_FOR_DYNACONF
  shadowing)
- Add upper bound to dynaconf version pin (>=3.2.13,<4.0)
- Tighten Pydantic Settings comparison to acknowledge 2.x TOML support
- Make .gitignore additions explicit in Phase 1 checklist
- Clarify that shell-level .env loading still works with load_dotenv=False

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -43,7 +43,7 @@ dependencies = [
     "pymupdf4llm>=0.2.2",
     "pymupdf-layout>=1.26.6",
     "openai>=2.8.1",
-    "dynaconf>=3.2.13",
+    "dynaconf>=3.2.13,<4.0",
 ]
 classifiers = [
     "Development Status :: 4 - Beta",