feat: add DeepPick, DeepOmit, DeepMerge non-strict type utilities

[kakasoo]

Summary

• Add DeepPick<T, K extends string>, DeepOmit<T, K extends string>: non-strict versions that accept any string key and silently ignore invalid keys (unlike DeepStrictPick/DeepStrictOmit which enforce valid keys at compile time)
• Add DeepMerge<Target, Source>: deep merge where Source takes precedence on overlap (spread/override pattern), unlike DeepStrictMerge where Target wins
• Add comprehensive tests: 38 new test cases covering valid keys, invalid keys, array handling, Date preservation, glob patterns, and type mismatches

Closes #11

Test plan

• npm run build:test && npm run test — all 367 tests pass
• npm run prettier — all files formatted

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