docs: define pull request label usage

[hetaoBackend]

Change

Document a small PR label scheme using the existing change-type and product labels. Keep perf:full as an independent performance-verification trigger, and point contributors to the rule from the contribution guide and PR template.

The currently open PRs were classified and labeled separately on GitHub. Storage-write PR #319 and the better-sqlite3 update PR #146 also received perf:full under the existing performance rules.

Validation

• git diff --check origin/main...HEAD: passed.
• pnpm verify --profile docs: passed all 4 applicable gates on macOS; release-tool tests had 51 passes and 1 platform-specific skip.
• Full runtime and cross-platform profiles were not run because this commit changes documentation and the PR template only.

Publication and contribution checks

• Existing files only; no source-inventory regeneration required.
• No credentials, account data, real user content, internal source history or private review material included.
• Shared-source port: not needed.

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