Iteration 5: export / import commands

Implements the `export` and `import` app-level commands per
ADR-0015 §11 + ADR-0007 amendment 1.

- `export [<path>]` writes a zip of project.yaml + data/ to
  <data-root>/YYYYMMDD-<projectname>-export-NN.zip by default,
  preserving the project's directory name as the single
  top-level folder inside the archive.
- `import <zip> [as <target>]` extracts an exported zip into
  a new named project and switches to it. Target name is
  derived from the zip's top-level folder by default; on
  collision the destination auto-suffixes -02, -03, ... up
  to -99 instead of refusing (deviates from §2's refuse-on-
  collision rule for save/save as; recorded as an amendment
  to ADR-0015 §11).
- Excludes playground.db and history.log from the zip.
- Path-traversal protection via zip::enclosed_name + post-
  resolution check that the extraction path stays inside
  the target directory.

Adds the zip = "5" dep with default-features = false +
features = ["deflate"] to keep the binary-size cost modest.

Test baseline: 370 passing, 0 failing, 0 skipped.

src/cli.rs

@@ -57,6 +57,13 @@ App-level commands (typed inside the app, available in both modes):
     load                     Open the project picker.
     rebuild                  Rebuild playground.db from project.yaml
                              + data/, with confirmation.
+    export [<path>]          Write a zip of project.yaml + data/ to
+                             <path> (relative paths under the data
+                             root). Excludes playground.db and
+                             history.log.
+    import <zip> [as <t>]    Unpack <zip> into a new project and
+                             switch to it. <t> overrides the target
+                             name (else taken from the zip).
 ";
 
 #[derive(Debug, thiserror::Error)]