chore(governance): add contributing and codeowners

Author: miccy

.github/CODEOWNERS

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+* @miccy

CONTRIBUTING.md

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+# Contributing
+
+## Development
+
+Requirements:
+
+- Bun (version pinned in `packageManager`)
+- Biome (workspace dependency)
+
+Setup:
+
+```bash
+bun install
+```
+
+Core checks:
+
+```bash
+bun run lint
+bun run check-types
+bun run test
+bun run verify
+```
+
+## Upstream Sync Rules
+
+- keep compatibility with `upstream/common-v8` unless an explicit fork decision exists
+- avoid dependency downgrades during sync work
+- keep fork-specific behavior isolated and documented
+- always run final gate before sync PR: `bun verify`
+
+## Pull Requests
+
+- keep changes focused by concern
+- include tests for behavior changes
+- update docs/knowledge when public API or workflow changes
+- do not commit secrets, tokens, or local credentials