ci(branching): adopt develop as the integration branch

[devin-ai-integration]

What and why

The repository moves to git-flow with two long-lived branches: develop is the integration branch every feature/, fix/, docs/, ci/ and test/ branch is cut from and targets; main carries releases only and receives release/x.y.z and hotfix/ pull requests. Tags (v* and the client package tags) are created on main, and after a release main is merged back into develop. This PR wires CI for that model and documents it. No Go code changes.

• .circleci/config.yml: the build-test workflow now runs on merges to both branches, so the race suite and the SonarCloud scan cover every integration merge. The &main-only anchor becomes &integration-branches with branches: only: [main, develop]; the release workflow stays on tags only. Workflow and proto-baseline comments name both branches — the baseline logic itself (first parent on a branch build, previous v* tag on a tag build) already works for either branch.
• Makefile: BUF_BREAKING_REF ?= origin/develop, since that is the branch an ordinary change merges into; comment and proto-breaking help text follow. A release/ or hotfix/ branch must compare against the released schema instead, so docs/project/releasing.md and the CONTRIBUTING.md versioning rule spell out make proto-breaking BUF_BREAKING_REF=origin/main for those branches; in CI the pull-request workflow already compares against github.base_ref, so a PR to main is checked against main.
• CONTRIBUTING.md: new Branches section stating the model; branch from / target develop; CircleCI runs "on every push to main or develop"; Creating a Release now goes through the release/x.y.z PR and the back-merge. The Release Checklist item on § Versioning is unchanged.
• docs/project/releasing.md: new The release branch section before Tagging (cut release/x.y.z from develop, fold fragments there with scripts/changelog.py release, PR to main, tag on main, merge main back into develop; hotfix/ follows the same path). The git checkout main && git pull && git tag … block is unchanged. The paragraph saying development happens on JPL-Devin/OpenSysML was no longer true — v0.5.0, v0.5.1 and v0.6.0 are tagged on Open-MBEE/OpenSysML main, and the recent history (release/0.7.0, docs(roadmap): add the bindings track, kernel conformance and the API element form #140–docs(roadmap): sweep the roadmap to main after v0.6.0 #149) is Open-MBEE branches — so it is rewritten: releases are tagged on Open-MBEE/OpenSysML, which is also where development happens, and the clients resolve releases from that repository.
• AGENTS.md: Golden Rule 3 and §6 step 1 diff against develop. Nothing else changed.
• README.md: the CI/CD line names both branches. The CircleCI badge stays on main.
• .github/workflows/pr.yml: header comment only; the workflow is keyed on github.base_ref and needs no filter change.
• changes/unreleased/git-flow-branches.changed.md: one user-facing entry.

SonarCloud. No sonar.branch.name or new-code setting was added. The sonarsource/[email protected] orb only downloads and runs sonar-scanner; SonarCloud lists CircleCI among its integrated CIs, for which the scanner sets sonar.branch.name and sonar.branch.target itself from the CI environment. sonar.projectVersion is still derived from the nearest v* tag, which develop inherits from main. Whether develop is analysed as a long-lived or a short-lived branch is decided by the project's long-lived-branch pattern in SonarCloud's project settings, which is a maintainer-side setting rather than something this config can express; the quality gate itself is unchanged.

Left on main on purpose — the published docs describe the released code:

• .github/workflows/pages.yml (branches: [main]), mkdocs.yml edit_uri: edit/main/docs/, and README.md "rendered from main".
• The README.md CircleCI badge (tree/main) and the blob/main / tree/main links in scripts/mkdocs_repo_links.py, scripts/mkdocs_landing.py, docs/guide/, docs/reference/ and clients/python/README.md.
• The .github/workflows/pr.yml comment "publishing a 404 from main" (it is about the Pages deployment).
• branch = "main" in the Rust git dependency examples (docs/guide/09-clients.md, docs/reference/rust-api.md, clients/rust/README.md): pinning the release branch is what a user wants.
• Historical measurements that name the main revision they were taken at (docs/project/performance-*.md, pilot-differential.md, pilot-xpect.md, grammar-coverage.md, validation-constraints.md, lossless-library-records.md, element-scoped-tier-gating.md, rdf-corpus-roundtrip.md, training-examples.md), and the staged plan in docs/internals/design/bounded-model-checking.md ("leaves main green"), which is a record rather than a policy statement.
• Non-branch uses: main packages and -X main.Version ldflags, Java src/main, Scoop's main bucket, "main entrypoints", an example branch = "main" in element-identity-annotations.md.
• docs/project/roadmap.md and docs/project/spec-compliance.md were not edited.

How it was verified

$ gofmt -l .                    # empty
$ go build ./...                # ok
$ go vet ./...                  # ok
$ make docs-check
python3 scripts/check-doc-links.py
0 broken link(s)
python3 scripts/check-doc-ids.py
No internal work-item labels in reader-facing documentation
python3 scripts/check-doc-figures.py
check-doc-figures: every quoted oracle figure names the round it measured
python3 scripts/changelog.py check
python3 scripts/mkdocs_census-test.py
Ran 6 tests in 0.018s
OK
$ python3 scripts/changelog.py check   # ok
$ python3 -c 'import yaml; yaml.safe_load(open(".circleci/config.yml"))'   # ok (the circleci CLI is not installed)

Loading the config with PyYAML also confirms the resolved filters: every build-test job carries {'branches': {'only': ['main', 'develop']}, 'tags': {'ignore': '/.*/'}} and the release jobs keep {'branches': {'ignore': '/.*/'}, 'tags': {'only': '/^v.*/'}}.

go test ./... was not run: no Go file changes.

Checklist

• make test and make lint pass locally — not run; no Go file changes (gofmt, go build, go vet pass)
• Tests added or updated for the change — n/a, CI configuration and documentation
• Documentation extended where it already covers the surface (see CONTRIBUTING.md)
• Changelog entry added as changes/unreleased/<slug>.<section>.md, not as an edit to CHANGELOG.md
• baselines regenerated and make docs-counts run if a gate count moved — n/a
• No internal work-item labels (waves, slices, F4, K5) in the body, docs, or changelog

[devin-ai-integration]

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