fix: align Cursor agent naming to use 'cursor-agent' consistently

The Python CLI was configured to use "cursor-agent" as the agent key in
AGENT_CONFIG, causing it to search for release packages with the pattern
"spec-kit-template-cursor-agent-sh-*.zip". However, the release build
scripts were generating packages named "spec-kit-template-cursor-sh-*.zip",
resulting in a mismatch that prevented successful template downloads.

This commit updates the release scripts to use "cursor-agent" consistently
throughout, aligning with the AGENT_CONFIG key and the documented best
practice of using actual CLI tool names as dictionary keys.

Changes:
- Update ALL_AGENTS array in create-release-packages.sh
- Update case statement for cursor-agent in build_variant()
- Update release asset paths in create-github-release.sh
- Update documentation in README.md and AGENTS.md to reflect correct usage

This ensures that `specify init --ai cursor-agent` correctly finds and
downloads the matching release package from GitHub.

Fixes the bug where cursor-agent initialization would fail with "No matching
release asset found" error.

**Written with the help of a cursor agent**