feat: initial parser for todo.txt format with spec and test cases

- Add parser for todo.txt tasks supporting priority, dates, projects,
  contexts, and key:value metadata
- Include full todo.txt format specification (SPECS.md) and visual
  description (description.svg)
- Add sample test.todo.txt file with valid, borderline, and malformed
  cases
- Initialize Go module and main entrypoint for parsing and JSON output
- Add .gitignore for binary artifacts

test.todo.txt

@@ -0,0 +1,90 @@
+# Valid tasks
+(A) Call Mom +Family @phone
+(B) Schedule Goodwill pickup +GarageSale @phone
+Post signs around the neighborhood +GarageSale
+@GroceryStore pies
+2011-03-02 Document +TodoTxt task format
+(A) 2011-03-02 Call Mom
+x 2011-03-03 Call Mom
+x 2011-03-02 2011-03-01 Review Tim's pull request +TodoTxtTouch @github
+(A) Call Mom due:2024-12-31
+x 2024-11-24 2024-11-20 Finish project due:2024-12-01 +Work @office
+
+# Valid metadata
+Buy milk due:2024-12-01
+Do taxes taxyear:2024
+Plan trip location:Rome
+
+# Valid multiple projects/contexts
+(A) Call Mom +Family +PeaceLoveAndHappiness @iphone @phone
+
+# Borderline: priority not at start
+Really gotta call Mom (A) @phone @someday
+(B)->Submit TPS report
+(b) Get back to the boss
+
+# Borderline: creation date not at start
+(A) Call Mom 2011-03-02
+
+# Borderline: completed but missing completion date (should error)
+x Call Mom
+
+# Borderline: project/context with only symbol
+Do something +
+Do something @
+
+# Borderline: key:value with protocol as key (should NOT be metadata)
+http://example.com
+https://example.com
+ftp://server.com
+mailto:someone@example.com
+
+# Borderline: key:value with protocol as key (should NOT be metadata)
+http:example.com
+https:example.com
+ftp:server.com
+mailto:someone@example.com
+
+# Borderline: key:value with value as URL (should be metadata)
+link:http://example.com
+website:https://example.com
+
+# Borderline: key:value with spaces (should NOT be metadata)
+foo:bar baz
+foo :bar
+foo: bar
+
+# Borderline: key:value with multiple colons (only first is split)
+multi:colon:example
+
+# Borderline: empty line
+   
+
+# Malformed: priority with lowercase letter
+(a) Call Mom
+
+# Malformed: completed with uppercase X
+X 2012-01-01 Make resolutions
+
+# Malformed: completed with x not at start
+(A) x Find ticket prices
+
+# Malformed: date with invalid format
+(A) 2024-13-99 Call Mom
+x 2024-99-99 Call Mom
+
+# Malformed: key:value with whitespace in key or value
+foo bar:baz
+foo:bar baz
+foo:bar:baz
+
+# Malformed: only symbols
++
+@
+:
+
+# Malformed: random text
+Just a random line with no special tokens
+
+# Malformed: only spaces
+