Review manifests drop valid capacity observations with Unix resets_at values

Summary

Review manifest generation silently discards otherwise valid capacity observations when resets_at is encoded as Unix seconds.

The frozen capacity format stores resets_at as Unix seconds, but the review planner only parses the field as RFC3339. As a result, latest_capacity_observation is reported as unavailable even though valid observations exist.

Reproduction

1. Index a valid capacity observation using the frozen format with a numeric Unix-seconds resets_at.
2. Generate a Review manifest for a scope that includes that observation.
3. Inspect latest_capacity_observation.

Observed result:

latest_capacity_observation: unavailable

A regression probe reproduced this against commit aad5070435fd9944e0cf38d32cba5d53e25425c4.

Expected behavior

The planner should decode every supported frozen resets_at representation and include the latest valid capacity observation in the Review manifest. Unsupported or malformed values should remain explicitly unavailable with a reason.

Relevant code

• RFC3339-only parsing in internal/reviews/plan.go

Acceptance criteria

• Numeric Unix-seconds resets_at values are decoded correctly.
• Existing RFC3339 values, if still supported, continue to work.
• A regression test verifies that a valid frozen capacity observation appears in latest_capacity_observation.
• Malformed reset values remain distinguishable from absent capacity data.

Start in internal/reviews/plan.go at the RFC3339-only parsing around line 144, then reproduce the review manifest case with a frozen capacity observation using numeric Unix-seconds resets_at. Add a regression test for latest_capacity_observation, confirming Unix-seconds and existing RFC3339 values are handled while malformed values remain distinct from absent data.