docs: add CONTRIBUTING.md and link it from READMEs

[zixuniaowu]

Summary

Adds a root-level CONTRIBUTING.md and links it from the English, Simplified Chinese, and Japanese READMEs.

The repository previously had no CONTRIBUTING file; contribution guidance was scattered across the README (4 bullet points), docs/guide/quality-gates.md, SUPPORT.md, and SECURITY.md. The new page consolidates these into one entry point for new contributors without duplicating detail — deep dives remain linked.

Contents

• Ways to contribute — issue etiquette from the README (existing issues first, required bug/feature details, CCCC_HOME boundary), plus the private-disclosure pointer to SECURITY.md
• Development environment — Rust 1.88 (rust-toolchain.toml), Node 24 (CI pin 24.19.0), Python 3.11+ (tooling only), and the source run command
• Embedded UI note — frontend changes require a Rust rebuild to appear in the native binary
• Project layout — brief crate/web/docs/scripts+tests map pointing at the architecture reference
• Quality gates — quality_gate.sh fast|full, pre_commit_checks.sh --dry-run, individual commands (matching CI invocations exactly), the serial daemon-test note, CCCC_CARGO_JOBS, the Connect browser acceptance gate, and the docs-contract-test caveat
• Code constraints — workspace lints (unsafe_code forbid; clippy unwrap_used/todo/dbg_macro denied) and the design-boundary stance on file length
• Commit messages — Conventional Commits with scopes; English or Chinese both accepted (matching existing history)
• PR process — fork/branch/verify flow and the CI jobs table from quality-gates.md
• Docs preview — VitePress dev command and the config.ts navigation requirement
• Community and license — Telegram link and the Apache-2.0 contribution statement

Verification

• All commands were cross-checked against .github/workflows/ci.yml, rust-toolchain.toml, and the [workspace.lints] table in Cargo.toml
• uv run --no-project --with pytest --with pyyaml python -m pytest -q — 121 passed
• scripts/quality_gate.sh fast — all impacted checks passed (8s)