chore: remove retired root legacy archive

[roger-gan]

Description

Remove the retired root legacy archive containing the previous Python SDK, old TypeScript SDK, superseded examples, e2e harnesses, and specifications. Update current documentation to point to the active TypeScript SDK and v2 specifications, while preserving the supported compatibility packages under typescript/packages/legacy.

Remove the PR template statements that incorrectly require GPG-signed commits, aligning contributor guidance with the actual branch protection settings.

Tests

• sh scripts/check_branch_policy.test.sh
• node --test scripts/check_pr_metadata.test.mjs
• pnpm format:check from typescript
• pnpm lint:check from typescript
• pnpm build from typescript
• pnpm test from typescript
• Verified all local links in changed Markdown files resolve
• Verified no tracked files remain under root legacy
• Verified all 164 tracked compatibility files remain under typescript/packages/legacy
• git diff --check

Branch route

• Normal development targets develop
• Only release_* or hotfix/* targets main

Checklist

• Formatting and lint checks pass
• All existing unit tests pass
• No publishable package changes; no Changeset required