feat: enable automatic deletion of merged head branches

[jonathanmorley]

Summary

• Enables GitHub's Automatically delete head branches (delete_branch_on_merge = true) for all 41 jonathanmorley repos (37 public, 4 private) by flipping the default in both public_default_settings and private_default_settings.
• Switches repo discovery queries from org:jonathanmorley to user:jonathanmorley — the account is a personal User account; both currently resolve identically but user: is the correct qualifier.

Plan verification (local)

• terraform validate: success
• terraform plan: 41 resources updated in-place, each with exactly delete_branch_on_merge: false -> true. No other attribute changes, no destroys or replacements (prevent_destroy also guards).

Enforcement after merge

Merging to main triggers the apply workflow (auto-approves the saved plan); the daily drift workflow keeps the setting enforced on all existing and newly created repos.