[guide] docs: document HIVE_SESSION multi-session contributor label (contributor-relay.md, env-vars.md)

[kubestellar-hive]

Documentation Fix

Documents the HIVE_SESSION multi-session contributor label added in b34254e ("contribute: allow multiple concurrent sessions per GitHub account"), which was previously discoverable only by reading bin/contributor-relay.sh.

Files/cluster claimed (docs only — no source code touched):

• src/docs/contributor-relay.md — new HIVE_SESSION row in the "Important environment variables" table + new section "Running multiple backends under one GitHub account" (default = backend name, same-backend labeling, empty-string opt-out, per-account limits shared across sessions).
• src/docs/env-vars.md — new HIVE_SESSION row in the "Contributor relay and top-level helper scripts" table, linking to the new section.

Disjoint from open hold-gated PRs #5559 (UPGRADE.md) and #4032 (proxy source).

Fixes #5605

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Filed by guide agent (ACMM L5 — hold-gated mode). Hold-gated: human review required.

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

[kubestellar-prow]

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[clubanderson]

Closing as a duplicate: this and #5613 both document the HIVE_SESSION multi-session label for #5605, and #5613 is the one moving forward. It carries the same table rows and multi-backend section, plus two things this PR misses:

1. The container path. Current v4 forwards HIVE_SESSION in neither the just contribute-hive docker run nor src/compose-contributor.yaml, so an explicit label set on the host is silently dropped in the default docker mode — docs: document HIVE_SESSION multi-session label for contributors #5613 adds the forwarding (empty-preserving ${VAR+...} form, since the empty string is the opt-out) so the documented examples are actually true.
2. The launch form. The AGENT_BACKEND=claude just contribute-hive example here does not select the backend: the recipe sources contributor.env with set -a before the ${AGENT_BACKEND:-claude} fallback, so the stored setup value clobbers the shell variable. The positional just contribute-hive claude form is the reliable one.

Your note that identical labels still collide on one task slot was a genuinely useful clarification and has been ported into #5613. Thanks — the gap was real, the surviving PR just closes it end to end.