fix(cli): render rule-history timestamps as dates, not a day count

[udsy19]

Summary

In the sandbox Rule History view, each entry's time renders as a days-since-1970 count (e.g. 19675d 22:13) instead of a wall-clock date.

Root cause

sandbox_draft_history formats event_time with format_timestamp_ms (crates/openshell-cli/src/commands/common.rs), which computes days = secs / 86400 and prints {days}d {hours:02}:{mins:02}. But event_time is an absolute google.protobuf.Timestamp (epoch ms), not an elapsed duration, so days is ~19,675. format_timestamp_ms was the only helper treating an absolute timestamp as a duration; the TUI's format_timestamp and the CLI's format_epoch_ms/format_optional_epoch_ms all render epoch-ms as YYYY-MM-DD HH:MM[:SS], and this was its sole caller.

Fix

Render with the existing format_optional_epoch_ms (as provider.rs does for event times), preserving the - placeholder for a missing/zero timestamp. The mis-implemented format_timestamp_ms helper is removed (no remaining references).

Tests

Regression test in common.rs: format_optional_epoch_ms(1_700_000_000_000) == "2023-11-14 22:13:20" and format_optional_epoch_ms(0) == "-". cargo test -p openshell-cli: 88 passed.

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