refactor: extract eval infrastructure to aikit-evals crate

[aroff]

Summary

• Moved the 6 generic eval modules (suite, config, runner, artifacts, checks, trace) from fastskill-evals into a new aikit-evals crate in goaikit/aikit (merged via PR feat: increase test coverage and improve CLI test suite #33)
• fastskill-evals is now a thin adapter: re-exports everything from aikit-evals and keeps only config_adapter.rs (reads [tool.fastskill.eval] from skill-project.toml)
• Fixed pre-existing flaky test failures in commands::add — three tests in init.rs were missing the DIR_MUTEX guard, causing set_current_dir races across threads

Dependency change

# Before (local path, dev-only)
aikit-evals = { path = "../../goaikit/aikit/aikit-evals" }

# After (git reference, aikit PR #33 merged at a22a619f)
aikit-evals = { git = "https://github.com/goaikit/aikit", branch = "main" }

Test plan

• cargo build --workspace — clean build against git dep
• cargo test -p fastskill-evals — 2/2 pass
• Pre-push hook: full 336-test suite passes (0 failures)
• Clippy clean across all crates

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