refactor: Use idiomatic let...else and remove redundant closures

[2witstudios]

Summary

• Replace match + early-return patterns with let...else in 7 locations across pu-core and pu-engine
• Replace redundant closures with method references (String::is_empty, ToString::to_string, IntoInnerError::into_error)
• No behavioral changes — pure idiomatic Rust modernization

Test plan

• cargo clippy --all-targets passes clean
• cargo test --workspace — all 405 tests pass
• cargo fmt --all — no formatting issues

🤖 Generated with Claude Code

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⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: c07d7b26-230b-421e-8a51-4221875cfefe

📥 Commits

Reviewing files that changed from the base of the PR and between 55aa882 and ed991c8.

📒 Files selected for processing (6)

• crates/pu-core/src/agent_def.rs
• crates/pu-core/src/manifest.rs
• crates/pu-core/src/schedule_def.rs
• crates/pu-core/src/swarm_def.rs
• crates/pu-core/src/template.rs
• crates/pu-engine/src/engine.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch pu/improve-1

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