ci: migrate default branch to main

[marianfoo]

Summary

• rename GitHub's stable integration/release branch from master to main
• move Quality and Release Please triggers to main, with an explicit Release Please target
• update current operational guidance while preserving historical branch references in dated evidence
• add a repository contract that prevents default-branch configuration drift
• switch and freshly verify A4H native abapGit repository 000000000017 on refs/heads/main

External migration state

• GitHub default: main at the exact former default head b6acc143e09590a48e3fef5b9ac3fda85834de0e
• remote master: absent after GitHub's supported rename
• local main: tracks origin/main; origin/HEAD points to origin/main
• Release Please PR chore(master): release 5.0.1 #23: automatically retargeted to main, preserved and unmerged
• A4H native abapGit: fresh read reports refs/heads/main; repository check is green
• NPL: remains offline; future reviewed ZIPs come from exact origin/main

Validation

• npm ci
• npm test (configured abaplint zero; repository and installation contracts green)
• git diff --check
• Node syntax check for the changed contract
• YAML parsing for both workflows
• complete branch-reference and sensitive-URL audits

SAP syntax, ABAP Unit, ATC, WebGUI, and ST22 are not applicable because this PR changes no src/ file, serialized SAP object, or live behavior.

Rollback

Use the same GitHub rename operation from main to master, revert the reviewed workflow/documentation commit, restore local tracking and origin/HEAD, and switch A4H native abapGit back with the verified full ref.

[marianfoo]

Final post-green audit complete on head 7b15b3c.

• Second Quality run 31197948092 is green.
• Repository Quality, abaplint, and abaplint observations all passed.
• The 15-file inventory matches the reviewed migration diff; there are no unresolved review threads.
• GitHub default, local tracking, origin/HEAD, Release Please PR base, and A4H native-abapGit full ref have all been verified as main.
• The remote master branch is absent.
• No src/ or serialized SAP object changed; SAP runtime gates remain correctly not applicable.

[marianfoo]

Post-merge verification complete.

• Squash commit: 973fcbd0a034cda8aba970a5326a40b2e7eb0d2a.
• Local main, origin/main, GitHub default, and origin/HEAD all agree.
• No remote master branch remains.
• Main-push Quality run 31198018520 passed.
• Main-push Release Please run 31198018392 passed.
• Release PR chore(main): release 5.0.1 #35 is the sole open PR, targets main, has the expected 5.0.1 patch, and Quality run 31198054392 is green.
• The identical pre-migration PR chore(master): release 5.0.1 #23 was closed without merge and its obsolete generated branch was deleted.
• A fresh ARC-1 read still reports A4H native-abapGit repository 000000000017 on refs/heads/main.
• Final local repository and sensitive-reference audits are green.