fix: implement KildError trait for DaemonClientError (#332)

[Wirasm]

Summary

DaemonClientError was the only error type in kild-core that didn't implement the KildError trait, breaking the documented error handling contract from CLAUDE.md.

Root Cause

The trait impl was simply never added when DaemonClientError was introduced in the initial daemon crate commit (6f1cfa7). All 14 other domain error types have KildError implementations.

Changes

File	Change
crates/kild-core/src/daemon/client.rs	Added KildError impl with DAEMON_* error codes
crates/kild-core/src/daemon/client.rs	Added tests for error_code() and is_user_error() on all 5 variants

Testing

• cargo fmt --check — clean
• cargo clippy --all -- -D warnings — 0 warnings
• cargo test --all — all 1,396 tests pass
• Error codes use DAEMON_* prefix matching DaemonAutoStartError convention
• Only NotRunning is marked as user error (user can fix by starting daemon)

Validation

cargo fmt --check && cargo clippy --all -- -D warnings && cargo test --all

Issue

Fixes #332

[Wirasm]

Automated Code Review

Summary

Clean implementation that follows established patterns exactly. No issues found.

Findings

Strengths

• Mirrors the exact KildError pattern from DaemonAutoStartError in daemon/errors.rs
• Error codes follow DAEMON_* prefix convention consistently
• Only NotRunning classified as user error — correct, since only that variant is user-actionable
• Tests cover all 5 variants for both trait methods

Security

• No security concerns — purely additive trait implementation with no new I/O or data handling

Checklist

• Fix addresses root cause (missing trait impl)
• Code follows codebase patterns
• Tests cover the change
• No obvious bugs introduced