refactor: rename tutorials/*/index.md to README.md for GitHub auto-rendering (#152)

GitHub only auto-renders README.md when browsing directories. Previously,
visiting tutorials/langchain-tutorial/ on GitHub showed a file listing —
users had to click index.md manually. Now the tutorial content renders
automatically.

Changes:
- rename 191 tutorials/*/index.md → README.md
- resolve 10 tutorials that had both files (delete old stub README.md,
  keep canonical index.md content, then rename)
- update 1,550 cross-references in chapter files: (index.md) → (README.md)
- update 14 Python scripts to reference README.md instead of index.md
- regenerate all discoverability assets, manifest, snapshots, status docs

Jekyll compatibility: GitHub Pages includes the jekyll-readme-index plugin
by default, which converts README.md to index.html. The has_children: true
frontmatter continues to work as before.

All CI checks validated locally:
- docs_health: 0 broken links, 0 missing indexes
- check_index_format_v2: 130 compliant indexes
- all diff checks (manifest, snapshot, repo status, discoverability): clean

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: johnxie

CONTENT_GAPS_ANALYSIS.md

@@ -60,7 +60,7 @@ High-impact surfaces requiring continuous maintenance:
 
 A tutorial track is considered production-ready when:
 
-- it has `index.md` with valid local links
+- it has `README.md` with valid local links
 - it has a coherent numbered chapter sequence
 - its summary and snapshot language are not stale or placeholder quality
 - it passes repository docs health checks

TUTORIAL_STRUCTURE.md

@@ -6,7 +6,7 @@ This file defines the canonical tutorial layout and current structure status.
 
 ```text
 tutorials/<tutorial-name>/
-  index.md
+  README.md
   01-*.md
   02-*.md
   ...

discoverability/query-coverage.json

@@ -7,7 +7,7 @@
 import re
 from pathlib import Path
 
-INDEX_NAME = "index.md"
+INDEX_NAME = "README.md"
 NAV_HEADING = "## Navigation & Backlinks"
 GENERATED_MARKER = "*Generated by [AI Codebase Knowledge Builder]"
 NUMBERED_FILE_RE = re.compile(r"^([0-9]{2,})[-_].+\.md$")