fix(skills): expire cache entries at deadline

[Sayt-0]

Summary

• treat cache entries as stale when the current time equals their expiry deadline
• prevent zero-freshness entries from being reused on coarse-resolution clocks such as Windows
• preserve existing behavior for entries whose expiry remains in the future

Context

The Windows test in Actions run 30802380958, job 91649724192 failed because TestDiskCache_Get_Expired received a cache hit for max-age=0.

Cache metadata is serialized to JSON, which removes Go's monotonic clock reading. On Windows, a subsequent wall-clock read can equal ExpiresAt. The previous strict After comparison treated that boundary instant as fresh. Using an inclusive expiry check implements now >= ExpiresAt semantics.

Validation

• go test -count=100 -run '^(TestDiskCache_Get_Expired|TestDiskCache_No(Store|Cache)StoresButExpiresImmediately)$' ./pkg/skills
• go test ./pkg/skills
• task test
• task lint
• task build
• task check-plan-cross
• Windows cross-compilation of the pkg/skills test binary