Name every get-todos --json field with its type in help and docs

[muellerei]

Summary

 get-todos --help, --json paragraph
-  Each task has marker, content (...), page and uuid; journal_day when ...;
-  scheduled, deadline, next_due (dates as YYYY-MM-DD), repeating,
-  references, references_withheld where they apply.
+  Each task has marker, content, page, uuid and hash, all strings: ...
+  Where they apply: journal_day (...), scheduled, deadline, next_due (all four
+  strings, YYYY-MM-DD), repeating (true), references (a list of page names:
+  journals newest first, then other pages, which a date range withholds) and
+  references_withheld (a number).

The --help epilog and the get-todos row in docs/commands.md name every --json field with its type. The epilog had left out hash. A caller read references as a list of objects and failed; it is a list of page names as strings. docs/design.md said references names days; it names pages.

Closes #129

Evidence

• Before: tests/test_get_todos_fields_documented.py against the old texts is red: the help misses hash, the reference row misses todos and count.
  After: green. Full suite 4049 passed, 8 skipped; ruff clean.

keys = every key in real get-todos --json output (mocked API)
for text in [--json paragraph of --help, --json paragraph of the docs row]:
  for key in keys:
    assert key is named in text

Removing any of the 15 keys from either text turns the test red (30 of 30).

Merge Danger

Door: two-way

Help text, docs and one test. The --json output does not change.

Blast Radius: none