chore: remove dead includes/pmid.php duplicate

[dknauss]

Removes includes/pmid.php — dead, unreachable code.

Why it's safe (verified):

• Never loaded — nothing requires/includes it; the main plugin file only requires the Composer autoload and the asset file.
• Registers nothing — only an ABSPATH guard + 5 function definitions (no add_action, no register_rest_route).
• Superseded — the live PMID resolver in bibliography-builder.php has refactored past it: a renamed caching API (…get_cached_pmid_result / …cache_pmid_result / …pmid_error) and renamed TTL constants (…_SUCCESS_CACHE_TTL, etc.). The dead file still calls the old …_response helpers and references old constant names.
• Latent fatal — if it were ever loaded it would throw Cannot redeclare function … plus undefined-constant errors.
• No references in composer.json, phpunit.xml.dist, or tests/; its only-there cache helpers are called solely from within the dead file itself.

Verification: composer test:php (109 pass), lint:php, npm test (550 pass), lint:js, npm run build — all green after removal.

This is the prerequisite cleanup referenced by the "leaner root plugin file" research backlog (#38).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.47%. Comparing base (710b7aa) to head (ee319c8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #39   +/-   ##
=======================================
  Coverage   81.47%   81.47%           
=======================================
  Files          40       40           
  Lines        2613     2613           
  Branches      464      464           
=======================================
  Hits         2129     2129           
  Misses        126      126           
  Partials      358      358           

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