refactor: count PR lines from the GitHub API instead of a git checkout

[NicolasMassart]

Summary

Fixes MCWP-747, where a retargeted pull request could be labelled from a count taken against its former base branch.

Rather than patching the base-branch resolution, this replaces the git-based counting with the pull request files endpoint, which reports per-file additions and deletions against the pull request's current base — the same three-dot diff shown in the Files changed tab. Only the pull request number now comes from the webhook payload, so a payload that lags a base retarget can no longer skew the count.

That removes every moving part the count previously depended on:

• actions/checkout, so the job no longer clones the repository
• the depth 1 → 10 → 100 → --unshallow fetch loop and the merge-base probing
• base-branch resolution, and with it the base-ref input

The action is now a single actions/github-script step, 240 lines down to 79, with no shell. Threshold and labelling behaviour is unchanged.

A pull request touching more than the 3000 files the endpoint lists is labelled size-XL and fails without counting lines, since it is over any configured limit regardless.

Compatibility

This ships under v1 as a non-breaking change.

Every caller invokes the action from a pull_request event, and none of them sets base-ref. Outside a pull request context the previous implementation already failed: the label step read context.payload.pull_request.number before its try block, so a branch run counted the lines, printed them, then threw a TypeError without ever reaching the pass/fail verdict. There is no working non-PR behaviour being removed.

The one new requirement is pull-requests: read, which callers granting pull-requests: write for labelling already have. Fork pull requests are unaffected: the token is read-only there, so the count succeeds and only the labelling is skipped, exactly as before.

contents: read is no longer needed in the calling workflow, though it is harmless to leave in place.

The workflow trigger fix that makes the check re-run on a base retarget is separate: MetaMask/metamask-mobile#33868.

Test plan

• Open a pull request and confirm the reported additions, deletions and size label match the Files changed tab
• Confirm files matching ignore-patterns are excluded from the total
• Retarget a pull request to a base with a different amount of divergence and confirm the recomputed label reflects the new base
• Confirm an existing, already-correct size label is left untouched, and a stale one is replaced without disturbing other labels
• Confirm a pull request over max-lines fails the check and is labelled size-XL

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