[Release] Configure release signing ownership and secrets

[jjoonleo]

Backlog ID: ISSUE-018
Source: plans/release_app_todos.md

Goal

Make release signing reproducible without exposing secrets.

Parallelization

Status: codex-ready

Prerequisites

• None

Acceptance criteria

• Release keystore owner and storage process are documented.
• Required local/CI inputs are documented: keystore file/path, store password, key alias, key password.
• android/key.properties or environment-variable path is documented without committing secrets.
• Release signing failure mode is checked and understandable.

Codex thread guidance

• Work only on this issue's scope.
• Do not modify unrelated release issues.
• If this issue is blocked by external access or human approval, prepare the needed checklist/text and leave the issue open.
• This issue is intended to be safe for an independent Codex thread now.