Add an ergo task to close the future --since validation gap.
Why: remote --since rejection and missing pool errors are currently skipped.
How: task calls for validating target and constraints before early exit.
Acceptance criteria cover remote/local follow/duplex behavior.
Preserves existing success exit for future --since on valid local pools.
Validation gates: cargo fmt --all, cargo clippy --all-targets, just ci-fast.
Constraints: keep CLI error messaging/hints aligned with existing helpers.
Risks/tests: verify remote --since rejection still fires after refactor.