DEVX-118: feat: enrich lint_docs.py with single H1, max depth, line length, code block lang, orphan checks

- Add check_single_h1: each markdown file should have at most one H1
- Add check_max_heading_depth: headings should not exceed H4 (configurable)
- Add check_line_length: warn on lines >120 chars (non-blocking — badge URLs)
- Add check_code_block_languages: fenced code blocks must specify a language
- Add check_orphan_docs: warn on docs not linked from index.md or mapping.json
- Fix all code blocks in docs to specify language (text for plain blocks)
- Fix duplicate H1 in .vale/styles/devx/README.md
- Add 18 new tests for full coverage of new checks

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

docs/tech/architecture.md

@@ -6,7 +6,7 @@ from scripts outside the package.
 
 ## Package structure
 
-```
+```text
 src/devx/
 ├── __init__.py              # Version (single source of truth, read by setuptools)
 ├── cli.py                   # Click-based CLI entry point (devx command)
@@ -439,7 +439,7 @@ v2 failures. Supports loading custom platforms from a JSON file.
 
 ### PR lifecycle
 
-```
+```text
 Developer creates Vikunja task (DEVX-N)
     │
     ▼
@@ -475,7 +475,7 @@ CI workflow (ci.yml) triggers:
 
 ### Post-merge flow
 
-```
+```text
 Push to master (squash-merge commit: "DEVX-N <conventional commit>")
     │
     ▼
@@ -519,7 +519,7 @@ Post-merge workflow (post-merge.yml) triggers:
 
 ### Publish flow
 
-```
+```text
 Tag push (vX.Y.Z) triggers publish workflow (publish.yml):
     │
     ▼
@@ -536,7 +536,7 @@ Tag push (vX.Y.Z) triggers publish workflow (publish.yml):
 
 ### Badge generation flow
 
-```
+```text
 push_badges.py:
     │
     ├── fetch_latest_master() → git fetch + reset --hard origin/master

docs/tech/ci-cd-workflow.md

@@ -6,7 +6,7 @@ tag-triggered publishing.
 
 ## Workflow overview
 
-```
+```text
 PR opened/synchronized ──► CI (ci.yml)
     │                         ├── quality
     │                         ├── detect-changes
@@ -143,7 +143,7 @@ updates.
 
 ### Job dependency graph
 
-```
+```text
 detect-type ──┬── validate-commit-msg (skip if release commit)
               ├── release (skip if release commit)
               │       │