refactor(template): optimize Rust code quality and add prek init (#42)

[crrow]

Summary

• paths.rs: Replace assert!/expect panics with Result-based init_data_dir() + snafu PathResolution variant
• app_config.rs: Replace unwrap_or_default() with snafu error propagation; new init()/get() two-phase pattern with idempotency guard
• backend.rs: Table-driven refactor replaces ~20 individual constructors with HEADLESS_PRESETS + INTERACTIVE_PRESETS static tables (745 → 521 lines)
• executor.rs: Unify terminate_child signature to &mut across unix/non-unix
• main.rs: Add init_data_dir() + app_config::init() startup sequence; load() → get(); section markers for config field registry
• justfile: Add prek-init, init recipes; setup-hooks now aliases prek-init
• tests/: Integration test scaffolding — common::cmd() helper + 5 smoke tests (help, version, agent-describe, hello, unknown command)
• agent-quickstart.md: Updated data flow diagram, module map, config access examples, new init/prek steps

Closes #42

Test plan

• Generate a test project with rara-cli-template setup
• cargo check compiles without errors
• cargo test — smoke tests pass
• just init — prek hooks install
• just pre-commit — all checks pass
• cargo run -- --agent-describe — valid JSON with protocol: "agent-cli/1"
• cargo run -- hello world — returns AgentResponse JSON

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