chore: enable ban-untagged-todo lint rule and tag existing TODOs

[tomas-zijdemans]

Enables the ban-untagged-todo lint rule, the second half of #7251. The style guide already requires TODOs to carry a username or issue link; this turns on the rule that enforces it.

Five untagged TODOs had accumulated. Each is now tagged with the username of the commit author who wrote it (via git blame): magurotuna (csv/parse.ts, csv/parse_stream.ts), cknight (dotenv/mod_test.ts), WWRS (testing/_snapshot_utils.ts, only needed a space removed), and 4513ECHO (toml/_test_utils.ts). No TODO text changed otherwise.

deno lint passes across all 1169 files with the rule on.

I used Claude Code to help investigate and write this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (ca58f94) to head (8caecb9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7293      +/-   ##
==========================================
- Coverage   95.03%   95.03%   -0.01%     
==========================================
  Files         617      617              
  Lines       51637    51637              
  Branches     9359     9359              
==========================================
- Hits        49075    49074       -1     
  Misses       2021     2021              
- Partials      541      542       +1     

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