Add repository baseline and licensing policy

[konojunya]

Background

As Render Toolchain Task 3, we will prepare the responsibility boundaries, license boundaries, and a minimal CI before implementation in an empty public repository. Required checks will be configured in the ruleset after verifying the actual workflow job names that succeeded.

Implementation Method

• Describe the responsibilities, non-responsibilities, and current status of the repository in the README
• Describe English operations, PR delivery, credential prohibition, and third-party asset acceptance criteria in AGENTS.md
• Apply Apache-2.0 to repository-authored work
• Add a ledger to track the provenance and individual licenses of third-party fonts/icons
• Add a minimal GitHub Actions workflow with the Theme repository job

Testing Method

• git diff --check "$(git hash-object -t tree /dev/null)" HEAD
• Workflow YAML parse
• Verify the existence and non-emptiness of README, AGENTS, LICENSE, and THIRD_PARTY_LICENSES
• Verify using SHA-256 that the LICENSE is the identical full text of Apache-2.0 used in existing Stack public repositories

Review Perspectives / Concerns

Only repository-authored free core themes are subject to Apache-2.0, while third-party assets retain their individual licenses. Paid or proprietary themes will not be placed in this public repository. Since the repository was empty, it uses bootstrap containing only an empty commit as its base. After merging, the default branch will be renamed to main.