Move docs and CI to main-only branching with a version bump per PR

[rickdijk]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

CI and documentation change: moves octo_image onto the main-only branching model already used by flutter_cache_manager and flutter_cached_network_image.

⤵️ What is the current behavior?

develop is the default branch and the branch of record. build.yaml triggers on develop. AGENTS.md, CONTRIBUTING.md and the PR template all point contributors at develop, and nothing enforces a version bump per PR. The README demo GIF is loaded from develop.

🆕 What is the new behavior (if this is a feature change)?

build.yaml triggers on main instead of develop. AGENTS.md and CONTRIBUTING.md describe main as the branch of record, with develop and master gone and a ruleset blocking develop's re-creation. Both documents, and the PR template, now require a pubspec.yaml version bump and a dated CHANGELOG.md entry on every PR, except docs-only and CI-only ones (this one included, so it doesn't bump). AGENTS.md's Releases section changes to tagging the merge commit directly, with no separate release-prep PR. The README GIF is pinned to the v2.1.0 tag instead of develop, so the link survives the branch move.

This PR only changes docs and CI config; main does not become the default or protected branch until a follow-up settings change once this is merged and green.

💥 Does this PR introduce a breaking change?

No.

🐛 Recommendations for testing

dart format --set-exit-if-changed ., flutter analyze and flutter test all pass locally with no code changes. Since build.yaml's pull_request trigger now filters on main and this PR targets develop, CI won't fire on the PR automatically; it was run via workflow_dispatch on this branch instead.

📝 Links to relevant issues/docs

FPL-29

🤔 Checklist before submitting

• All projects build
• Follows style guide lines
• Relevant documentation was updated
• Rebased onto current main
• Version bumped in pubspec.yaml and dated CHANGELOG.md entry added (skip for docs-only and CI-only changes)

🤖 Generated with Claude Code