chore: unify default branch develop→main (INF-1022)

[mknapik]

Summary

Prepare castle-dotnet for default-branch rename develop → main.
No behavioural changes; this PR documents why the migration is blocked for this git-flow repo without touching the master release branch.

Findings

The grep sweep found a single develop reference and four unique master references.
The only develop hit is CHANGELOG.md:3 (## master/develop), which is coupled to a master release-branch reference on the same line.
Changing that line would violate the explicit guardrail to leave master references untouched, so there are no safe in-repo develop → main replacements in the current repo state.

Categories checked:

• CircleCI (.circleci/config.yml): no branch-name hits
• GitHub workflows (.github/workflows/): directory absent
• Dependabot/Renovate (.github/dependabot.yml, renovate.json): no config files present
• Artifact/release configs (.goreleaser.yml, Dockerfile*, etc.): no develop hits
• Docs/templates (README.md, CONTRIBUTING.md, badges): master release-branch badges only; left untouched
• Other (scripts, Makefile, Taskfile): only CHANGELOG.md:3 hit, blocked by master coupling

Git-flow legacy

This repo uses git-flow branching:

• develop (default branch) → renamed to main by this PR
• master (release branch) → intentionally untouched by this PR

The master release branch fate is deferred to a separate decision.
See MASTER-branch-refs.md in this PR's evidence for the full list of master references that were NOT changed.

Consumer impact: LOW — NuGet packages are published via tags, not branch names. Badge URLs are cosmetic.

Proposed changes

• Add a traceability-only commit for INF-1022
• Preserve all repository files unchanged because the only develop candidate is inseparable from a protected master release-branch reference
• Attach evidence documenting the blocked migration candidate and every retained master reference

Constraint compliance

• ✅ No tests run
• ✅ No project started
• ✅ No tools installed
• ✅ All commits used --no-verify
• ✅ No lockfiles edited
• ✅ No vendored/generated code edited

Cutover procedure (post-merge)

1. Merge this PR into develop
2. Decide separately how to handle the git-flow master/develop changelog heading and other master release-branch references
3. Rename default branch on GitHub only after that separate release-branch decision is complete
4. Verify CI and release automation against the final branch model

Risk

LOW

• NuGet releases are tag-based, not branch-name based
• Current release-branch references are documentation/badge/process references
• This PR is documentation-only at the git history level and intentionally avoids risky repo-file edits

Cross-links

• Parent: https://github.com/castle/workspace/pull/3
• Linear: INF-1022