[rwgk]

— codex GPT-6 on behalf of Ralf —

Layer 7 of 8. Previous: #3177. Next: #3182. Stack map at the bottom.

What this PR does

Writes down how both bindings majors are maintained on main, and retires the 12.9.x-branch automation.

• Policy (.github/RELEASE-bindings.md, new; ci/README.md; cuda_bindings_12/MAINTENANCE.md, new): develop both lines on main; cut a short release branch only to stabilize or for an urgent fix, and return every fix to main; the 12.9.x branch is frozen except for emergency fixes with its historical tooling; major-line maintenance may include selected compatibility work, a frozen patch release admits only approved fixes; record scope in the checklist and notes.
• Backports: backport.yml becomes manual-only (a merged PR number plus an existing target branch, validated); the automatic to-be-backported trigger and dependabot's 12.9.x entry are removed.
• Registry: backport_branch leaves ci/versions.yml together with its last consumer.
• Guidance: AGENTS.md gains the rules for SYNCED code blocks; CONTRIBUTING.md describes release branches; package AGENTS.md files link to the policy.

After this merges

• Nothing changes in builds or tests. Releases follow RELEASE-bindings.md.
• Two follow-ups stay outside the stack by design: push the first retained milestone tag v12.9.11a0 on validated main history, then delete the version preview bootstrap; and remove compatibility notes once no older artifact sources remain.

Where to look

• .github/RELEASE-bindings.md: the policy reviewers are really approving.
• .github/workflows/backport.yml: 100 lines, manual-only.
• ci/README.md: "Maintaining Both Bindings Roots" and "Changing Supported Release Lines".

All changes (15 files, +349 / -137)

.github/RELEASE-bindings.md (new), .github/RELEASE-core.md, .github/dependabot.yml, .github/workflows/backport.yml, AGENTS.md, CONTRIBUTING.md, ci/README.md, ci/tools/bindings_config.py (drop the backport_branch acceptance), ci/versions.yml, cuda_bindings_12/{AGENTS.md,MAINTENANCE.md (new),README.md,docs/README.md}, cuda_core/AGENTS.md, cuda_python/AGENTS.md.

Validation

• Policy and automation only; no package runtime behavior changes. The complete stack passed full CI at cuda.bindings: backport CUDA 12 runtime maintenance fixes #3182's pre-rebase head 341d286d (run 40997121153, attempt 3, 105 jobs).
• Not exercised: the manual backport workflow against a live target branch.
• Current head 18df0cd6418cfd0c686186eda85cc1dab402c943 includes the follow-up fixes. Cumulative validation at cuda.bindings: backport CUDA 12 runtime maintenance fixes #3182: 349 CI-tool tests and full pre-commit passed. Full CI and native Pixi source/GPU checks have not been rerun for it.

Stack map: review scopes and merge units

Layer	PR	Scope	Merge unit
1	#3172	import cuda_bindings_12/, byte-identical to the 12.9.x branch	merged
2	#3173	integrate the import with the shared build tooling	merged
3	#3174	registry, packaging, version selectors, developer environment, guidance	group: #3174 + #3175 + #3176 land together
4	#3175	build both majors from main; planner; version preview bootstrap	group
5	#3176	test, document, nightly, coverage; first CI-complete state	group
6	#3177	releases driven by the tagged source's configuration	after the group
7	#3178	maintenance policy, manual backport workflow, retire backport_branch	after #3177
8	#3182	three CUDA 12 runtime fixes	independent of the CI layers; stacked only for ordering

Layers 3 to 5 are review boundaries, not working intermediate states: #3174 alone breaks CI (workflows still read the removed cuda.build.version), and #3175 alone fails actionlint (callers pass an input the test workflows only accept in #3176). The plan is to review them separately and land the reviewed group as one merge. #3176 is the full-CI target for the group.

The staged stack #3314-#3322 is the alternative in which every layer is mergeable on its own; both stacks reach the same end state, except that the staged stack keeps compatibility routes for older artifact sources and retained CUDA 13 tags.

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[rwgk]

/ok to test 9cf3778

[github-actions]

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[rwgk]

/ok to test fb5cced