New Pattern: Add Centralised InnerSource Repository Governance pattern

[amburi]

Summary

Adds a new InnerSource pattern for maintaining repository contributor readiness at scale through a central
governance repository, policy-as-code, and scheduled readiness audits.

Changes

• Introduces the Centralised InnerSource Repository Governance pattern.
• Frames governance around InnerSource readiness, discoverability, documentation, ownership, communication,
  and contribution paths.
• Defines readiness policies for checks such as README.md, CONTRIBUTING.md, CODEOWNERS, issue/PR templates, labels, portal metadata, and trusted collaboration settings.
• Adds guidance for repository inventory, policy profiles, exception handling, and source-of-truth
  boundaries.
• Covers incremental, rate-aware audits to reduce GitHub API and CI infrastructure strain.
• Clarifies roles for contributors, Trusted Committers, repository maintainers, ISPO/community stewards,
  platform/security partners, and Product Owners.
• Includes governance repository protection, reporting principles, remediation guidance, and operating
  metrics.
• Links related InnerSource patterns.