Point source-repository head at main

[alunduil]

Summary

One line: source-repository head in collection-json.cabal named develop,
which #82 renamed to main and retired. Now names main.

Closes #266.

Gotchas

The reason this is worth a PR of its own rather than a drive-by in the release
commit: the stanza is not inert metadata. It ships in the sdist, renders on the
Hackage page, and is what cabal get -s clones. Against the currently published
version it fails outright:

$ cabal get -s collection-json-1.3.1.3
Cloning into 'collection-json'...
fatal: Remote branch develop not found in upstream origin
Failed to fetch the source repository for package collection-json-1.3.1.3,
repository location https://github.com/alunduil/collection-json.hs
(git failed with ExitFailure 128).

The published 1.3.1.3 cabal file carries the same branch: develop, so this
broke when the branch was renamed rather than at any upload — it is live on
Hackage today, not a prospective problem.

Sequenced ahead of the 1.3.2.0 tag (#84) so that release is the upload that
fixes the pointer instead of the second one shipping it broken.

Verification

• cabal check — no errors or warnings.
• git ls-remote --heads https://github.com/alunduil/collection-json.hs main
  resolves, so the branch the stanza now names exists.
• Built the sdist and confirmed the extracted collection-json.cabal carries
  branch: main — the fix reaches the artifact, not just the working tree.
• pre-commit run --files collection-json.cabal passes.

cabal get -s itself can only be exercised post-publish, so the ls-remote and
sdist checks stand in for it until 1.3.2.0 is up.

🤖 Generated with Claude Code

https://claude.ai/code/session_013U3Mvn9yPinG1dRj8p5DEv

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.46%. Comparing base (b82a39c) to head (26dbc81).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #267   +/-   ##
=======================================
  Coverage   77.46%   77.46%           
=======================================
  Files           1        1           
  Lines         142      142           
  Branches       27       27           
=======================================
  Hits          110      110           
  Misses          5        5           
  Partials       27       27           

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