fix(release): round-trip commit dates in plan storage

[jasonkuhrt]

Closes #184

Summary

• make the shared git Commit schema round-trip canonical ISO 8601 UTC date strings instead of failing when release plans are reloaded from JSON
• add release-plan and git-level regression coverage for commit date round-trips and reject non-canonical date strings
• wire @kitz/git into the workspace test:packages entrypoint so the new regression coverage runs in normal package test sweeps

Reproduction

• bun run --cwd packages/release test packages/release/src/api/planner/models/plan.test.ts

Verification

• bun run --cwd packages/git check:types
• bun run --cwd packages/git test
• bun run --cwd packages/release test packages/release/src/api/planner/models/plan.test.ts
• bun run release:verify
• bun run test:packages

Review

• Subagent review completed and follow-up findings were addressed before merge.

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