[strategist] planning: release-channels.md — document channel divergence (v4→stable/candidate, v5→edge) (Refs #5687)

[kubestellar-hive]

What

Updates src/docs/release-channels.md only (the promotion-policy note, the "How channels are published" intro, the branch-restriction line, and the monotonic-tags parenthetical) to reflect current CI reality:

• v4 docker.yml publishes CHANNELS: "stable,candidate" (three image jobs);
• v5 docker.yml publishes edge (publish-image-tags.sh … v5 true edge);
• so edge is now an active-development v5 build, not a synonym for stable.

The doc previously said all three channels retag on every merge to v4 and co-point with v4-latest — contradicting both the workflows and ROADMAP.md's "divergence has begun". The remaining forward-looking caveat (soak/promotion between candidate and stable within the v4 line, #3702) is retained.

Refs #5687

Files/cluster claimed

src/docs/release-channels.md only. Disjoint from open hold PRs #5673 (contributor-relay.md), #5665 (labelPlanSink tests), #5642 (ADOPTERS.md), #5635 (docs/rfc-4002-phased-roadmap.md), #5559 (UPGRADE.md), #4032 (proxy auth).

---

Hold-gated planning PR by strategist agent (ACMM L5). Do not merge without human review; never remove the hold label.

— hive: agent=strategist backend=copilot model=claude-sonnet-4-6 copilot=1.0.78

[clubanderson]

Review — OK (accurate; hold-gated, leaving the hold in place for human sign-off).

Verified the changed claims against both workflows:

• v4 docker.yml: RELEASE_BRANCH: "v4" + CHANNELS: "stable,candidate" in all three image jobs and the cross-org mirror steps — matches "every merge to v4 retags stable and candidate".
• v5 docker.yml: all three images invoke publish-image-tags.sh … 'v5' {true|false} 'edge' — matches "every merge to v5 retags edge", so "edge is now an active-development v5 build" is exactly right.
• The generalized branch-restriction line ("release branches (v4, v5)") and the monotonic-tags parenthetical ("the branch's -latest tag and its channels") both match the per-line publish gating.
• Retaining the ✨ ci: publish stable/candidate/edge release channels from v4 #3702 caveat narrowed to candidate/stable within the v4 line is the correct residual — that soak step still doesn't exist.

No code change implied: src/pkg/hub/release_channels.go deliberately derives channel→branch by digest resolution rather than a hardcoded mapping, so the pill will start reporting edge (v5) through the existing mechanism with no hub change — the doc stays honest either way.

One non-blocking nit outside the changed hunks, for a follow-up rather than this PR: the "Switching a hive to a channel" section still describes switch-completion as heartbeat tag-match (#3761). Given the upgrading-beat payload is minimal (#5585) and moving-tag deployments are only truly verifiable by digest, that section would benefit from a digest-verification caveat.

[kubestellar-prow]

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