give Comment a timestamp, display in chronological order

This commit makes a couple of changes:

- `ent show ISSUE` now displays the Issue's Comments in chronological
  order

- the Comment struct now includes a timestamp, which is the Author Time
  of the oldest commit that touches the comment's directory

- the Issue struct now stores its Comments in a sorted Vec, not in
  a HashMap

- The Comment's uuid moved into the Comment struct itself, instead of
  being the key in the Issue's HashMap of Comments

Cargo.toml

@@ -9,6 +9,7 @@ log = ["dep:log", "dep:simple_logger"]
 
 [dependencies]
 anyhow = "1.0.95"
+chrono = "0.4.41"
 clap = { version = "4.5.26", features = ["derive"] }
 log = { version = "0.4.27", optional = true }
 rand = "0.9.1"