feat: add Rust CLI implementation

[nmccready]

Summary

• Add Rust CLI (envcache) with full feature parity to the shell version
• 4 modules: config, secrets, cache, resolve — matching shell script behavior exactly
• 13 unit tests covering secrets parsing, cache lifecycle, permissions, and refresh triggers
• Multiple output formats: --format export (shell eval), --format dotenv, --format json
• CI updated: tests.yml runs shell (bats) and Rust (fmt + clippy + test) in parallel
• rust-release.yml workflow for cross-compiling binaries (linux/macOS, amd64/arm64) and uploading to GitHub Releases on tag

Usage (Rust binary)

# Resolve and cache (outputs export statements for eval)
eval "$(envcache)"

# Force refresh
eval "$(envcache --force)"

# Custom paths
envcache --secrets-file ./my-secrets --ttl 3600

# JSON output
envcache --format json

Architecture

src/
├── main.rs      — CLI (clap), output formatting
├── config.rs    — Config struct, defaults
├── secrets.rs   — Parse secrets file, hash for change detection
├── cache.rs     — Read/write cache + meta, freshness checks
└── resolve.rs   — Call `op read`, post-processors

Test plan

• cargo fmt --check passes
• cargo clippy -- -D warnings passes (zero warnings)
• cargo test — 13/13 pass
• npm run lint — shellcheck clean
• npm test — 10/10 bats tests pass
• CI runs both shell and Rust test suites in parallel

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