Add release-plan.yaml for automated release tracking

TL;DR: This PR adds release-plan.yaml for automated release tracking (replacing manual API Release Tracker pages on wiki).

Note: Adding release-plan.yaml is a prerequisite for the upcoming release automation process.
This PR does not yet enable the automated release workflow; onboarding will follow separately.

What is this?

The release-plan.yaml file declares your release plan for this repository and its APIs. It enables:

• Automated release tracking (replacing manual API Release Tracker pages on wiki)
• CI validation of release readiness
• Automated release preparation (enabled during onboarding)

Pre-populated data

The release-plan.yaml file is generated from your current release data and repository settings:

• Release tag: target_release_tag is set to r1.3 (detected from your current release)
• APIs: derived from your current release (r1.3)
• Contacts: from your CODEOWNERS file

👉 Please review and adjust if API-specific contacts differ from repository-wide codeowners.

What to do next

Option A: Merge as-is (if no release planned yet)

• The file reflects your current release state (r1.3, Fall25)
• target_release_type: none means "no release currently planned"
• Just merge

Option B: Update before merging (if planning your next release now)

1. Set target_release_type (e.g. pre-release-alpha or pre-release-rc)
2. Update target_release_tag for your next release:
   • New cycle: increment first number, reset second to 1 (e.g., r3.2 → r4.1)
   • Progression in same cycle: increment second number (e.g., r3.2 → r3.3)
3. Update target_api_version for any APIs being changed
4. Optionally add meta_release: Sync26 if participating in the meta-release

Documentation

📖 Release Management Documentation
📖 The release-plan.yaml File
📖 Release Lifecycle
📖 API Versioning