feat: Add LanguageTool integration for comprehensive grammar checking

Integrated LanguageTool as an on-demand grammar checker, replacing the
previously disabled proselint checker.

Changes:
- Created scripts/languagetool-flycheck wrapper for flycheck integration
  - Converts LanguageTool JSON output to flycheck format
  - Includes suggestions in error messages
  - 30-second timeout for large files

- Updated modules/flycheck-config.el:
  - Defined languagetool checker for text/markdown/org/gfm modes
  - Updated cj/flycheck-prose-on-demand to use LanguageTool
  - Added installation instructions (sudo pacman -S languagetool)
  - Improved documentation clarity

- Usage: Press C-; ? in org/text/markdown files
  - Enables flycheck with LanguageTool
  - Shows errors in *Flycheck errors* buffer
  - On-demand only (no performance impact)

- Updated docs/NOTES.org:
  - Added best practice: Test Emacs launch after non-trivial changes
  - Example: emacs --eval "(kill-emacs)"
  - Catches startup errors before committing

- Disabled weather debug mode (wttrin-debug nil)

- Marked todo.org grammar checker task as DONE

LanguageTool catches real grammar issues (subject-verb agreement, tense,
punctuation, common mistakes) that proselint missed.

Installation: LanguageTool 6.6 (222MB) from Arch repos
Dependencies: Python 3 (for wrapper script)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

1 files changed, 82 insertions, 0 deletions

diff --git a/scripts/languagetool-flycheck b/scripts/languagetool-flycheck
new file mode 100755
index 00000000..ecbc900f
--- /dev/null
+++ b/scripts/languagetool-flycheck
@@ -0,0 +1,82 @@
+#!/usr/bin/env python3
+"""
+Wrapper for LanguageTool to produce flycheck-compatible output.
+Output format: filename:line:column: message
+"""
+
+import json
+import sys
+import subprocess
+
+def main():
+    if len(sys.argv) < 2:
+        print("Usage: languagetool-flycheck FILE", file=sys.stderr)
+        sys.exit(1)
+
+    filename = sys.argv[1]
+
+    # Run languagetool with JSON output
+    try:
+        result = subprocess.run(
+            ['languagetool', '-l', 'en-US', '--json', filename],
+            capture_output=True,
+            text=True,
+            timeout=30
+        )
+    except subprocess.TimeoutExpired:
+        print(f"{filename}:1:1: LanguageTool timeout", file=sys.stderr)
+        sys.exit(1)
+    except Exception as e:
+        print(f"{filename}:1:1: LanguageTool error: {e}", file=sys.stderr)
+        sys.exit(1)
+
+    # Parse JSON output
+    try:
+        # Find the JSON in the output (skip warning lines)
+        json_output = None
+        for line in result.stdout.split('\n'):
+            if line.startswith('{'):
+                json_output = line
+                break
+
+        if not json_output:
+            sys.exit(0)  # No errors found
+
+        data = json.loads(json_output)
+
+        # Read file to calculate line numbers from character offsets
+        with open(filename, 'r', encoding='utf-8') as f:
+            content = f.read()
+
+        # Convert matches to flycheck format
+        for match in data.get('matches', []):
+            offset = match['offset']
+            length = match['length']
+            message = match['message']
+            rule_id = match['rule']['id']
+
+            # Calculate line and column from offset
+            line = content[:offset].count('\n') + 1
+            line_start = content.rfind('\n', 0, offset) + 1
+            column = offset - line_start + 1
+
+            # Get first suggestion if available
+            suggestions = match.get('replacements', [])
+            if suggestions:
+                suggestion = suggestions[0]['value']
+                message = f"{rule_id}: {message} Suggestion: {suggestion}"
+            else:
+                message = f"{rule_id}: {message}"
+
+            # Output in flycheck format
+            print(f"{filename}:{line}:{column}: {message}")
+
+    except json.JSONDecodeError as e:
+        print(f"{filename}:1:1: Failed to parse LanguageTool JSON: {e}", file=sys.stderr)
+        sys.exit(1)
+    except Exception as e:
+        print(f"{filename}:1:1: Error processing LanguageTool output: {e}", file=sys.stderr)
+        sys.exit(1)
+
+if __name__ == '__main__':
+    main()