Remove legacy Python runtime

[ZHOUKAILIAN]

Summary

• Remove the legacy Python runtime, Python packaging files, and Python unittest suite.
• Move CI and release flows to Node/npm only, including npm tarball release packaging and a Node-based release helper.
• Update current docs/scripts/server fallback paths so the JS runtime is the only active implementation.

Test Plan

• npm run typecheck
• npm run build
• npm test
• node packages/cli/dist/index.js run "merge smoke quick js runtime" --profile quick --repo-root . --state-root /tmp/agt-js-merge-smoke-state
• node packages/cli/dist/index.js status --state-root /tmp/agt-js-merge-smoke-state
• node packages/cli/dist/index.js migrate --from . --state-root /tmp/agt-js-merge-migrate-state --dry-run
• node scripts/release/release-tools.mjs verify-version --tag v0.3.0-alpha.1
• node scripts/release/release-tools.mjs release-type --version 0.3.0-alpha.1 --github-args
• npm pack --pack-destination /tmp/agt-js-merge-release
• local install.sh smoke against file:///tmp/agt-js-merge-release
• residual scan for Python runtime/package/test entrypoints