Use more idiomatic Effect parsing helpers

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What Changed

• Replaced the VCS action target key JSON.parse/JSON.stringify path with a hoisted Schema.fromJsonString codec for symmetric encode/decode.
• Converted CLI duration parsing to return Option<Duration> instead of Duration | null.
• Added focused @effect/vitest coverage for the Schema JSON key round trip and duration shorthand/invalid parsing.

Why

These are small Effect idiom improvements that keep parsing behavior centralized in Effect Schema and model missing parse results with Option instead of nullable values.

UI Changes

None.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

  

Note

Use Effect schema helpers for duration and VCS action target key parsing

• Refactors parseDurationInput in config.ts to return Option<Option<Duration>> and replaces null checks with Option.none/Option.some throughout.
• Introduces VcsActionTargetKeyJson in vcsAction.ts as a Schema.fromJsonString codec for target keys, replacing manual JSON.stringify/JSON.parse calls.
• Adds encodeVcsActionTargetKey and updates decodeVcsActionTargetKey to use schema-based encoding/decoding consistently.
• Adds tests for duration shorthand decoding, rejection of invalid durations, and a round-trip test for VCS action target keys.

^{Macroscope summarized 311a8f7.}