documentation: link docs/branch-protection.md from the docs map

[codeforester]

Goal

Make docs/branch-protection.md discoverable from the repo's normal
documentation entry points.

Background

Found during a pre-release documentation sweep (2026-09-21), reviewed against
origin/main (b461b5a, "ci: add stable consumer and compatibility gates
(#39)"). docs/branch-protection.md documents the required branch-protection
status checks (required-consumer, required-compatibility) and the
single-maintainer review-approval exception. It is a real, substantive
(22-line) operational runbook, added alongside the CI gates it describes, but
it is not linked from README.md, CONTRIBUTING.md, or docs/README.md —
nothing else in the repository references it by name. A maintainer setting up
branch protection (or #27, "ci: require consumer and compatibility checks
before merging demo changes," which this doc appears to support) would only
find it by browsing docs/ directly.

Scope

• Add a link to docs/branch-protection.md from docs/README.md (the
  "Documentation guide") and/or CONTRIBUTING.md, wherever repository
  maintenance/ops docs are otherwise pointed to.
• No content change to docs/branch-protection.md itself is required unless
  the linking pass surfaces something stale.

Acceptance Criteria

• docs/branch-protection.md is linked from at least one of
  docs/README.md or CONTRIBUTING.md.
• git diff --check passes.

Validation

git diff --check

Non-Goals

• Not changing the branch-protection configuration or the CI gates
  themselves.

Project Fields

• Status: Backlog
• Priority: P3
• Size: T
• Area: Docs
• Initiative: Adoption Polish
• Milestone: v0.1.0