Fix fork GitHub Actions failures

[iamxvbaba]

What changed

• derive imported schema paths with Git's repository-relative prefix so macOS path aliases and Windows 8.3 paths resolve consistently
• enforce LF checkout bytes for all TL schema files
• raise the race package timeout from 5 to 15 minutes
• keep coverage generation and its artifact, but remove the upstream Codecov upload that cannot authenticate for this fork
• remove the credential-dependent scheduled schema updater; fork schema updates remain a reviewed local import and policy-audit workflow

Root cause

The fork inherited upstream automation assumptions that do not hold here: Codecov and GitHub App secrets are absent, and automatic schema update PRs conflict with the fork's selective-sync policy. The remaining CI failures came from OS-specific path spellings, CRLF checkout bytes changing locked schema hashes, and the sparse model tests exceeding the race build's five-minute package timeout.

Impact

The useful E2E, benchmark, check, coverage, Linux, Windows, macOS, oldstable, stable, and race gates remain enabled. Only external publishing and automatic schema mutation are removed.

Validation

• go test ./cmd/gotdgen ./gen/semantic -count=1
• go test --timeout 15m ./...
• go generate . with zero generated-file diff
• git diff --check