Only enforce codecov check on pull requests

[marlo-longley]

This should prevent a red x that we frequently see on the main branch / repository homepage -- making the project-level check information-only.

the resulting status will pass no matter what the coverage is or what other settings are specified.

For PRs ("patches" in codecov) CI will still break if tests decrease below threshhold.

Also from my reading of the docs and using codecov's validator service, it seemed like they wanted me to rewrite the key structure, removing top level codecov:

curl --data-binary @.codecov.yml https://codecov.io/validate
Valid!

{
  "coverage": {
    "precision": 1,
    "round": "up",
    "status": {
      "project": {
        "default": {
          "informational": true
        }
      },
      "patch": {
        "default": {
          "informational": false
        }
      }
    }
  },
  "ignore": [
    "^vite.config.js.*"
  ]
}

vs.:

 curl --data-binary @.codecov.yml https://codecov.io/validate
Error at ['codecov', 'coverage']: 
unknown field

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.2%. Comparing base (feb1375) to head (957a9f9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4171     +/-   ##
=======================================
+ Coverage   95.0%   95.2%   +0.3%     
=======================================
  Files        325     325             
  Lines      16153   16153             
  Branches    2535    2547     +12     
=======================================
+ Hits       15343   15376     +33     
+ Misses       805     772     -33     
  Partials       5       5             

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