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 (crates/openshell-cli/src/run.rs) formats each entry's event_time with format_timestamp_ms (crates/openshell-cli/src/commands/common.rs):

let days = secs / 86400;
if days > 0 {
    format!("{days}d {hours:02}:{mins:02}")
} else { ... }

But event_time is an absolute google.protobuf.Timestamp (epoch ms, via proto_timestamp_ms → timestamp_to_millis), not an elapsed duration — so days is ~19,675 and every entry prints garbage like 19675d 22:13. format_timestamp_ms was the only helper in the crate that treated an absolute timestamp as a duration; the TUI's format_timestamp and the CLI's format_epoch_ms/format_optional_epoch_ms all correctly render epoch-ms as YYYY-MM-DD HH:MM[:SS], and this was its sole caller.

Fix

Render the absolute timestamp with the existing format_optional_epoch_ms (same helper provider.rs uses for event times), which also preserves the - placeholder for a missing/zero timestamp. The mis-implemented format_timestamp_ms helper is removed (no remaining references).

Tests

Added a 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) == "-". Verified with cargo test -p openshell-cli --lib (fails on the old helper with 19675d 22:13, passes after).

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