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/action.rs

@@ -59,4 +59,27 @@ pub enum Action {
     NewProject {
         source: String,
     },
+    /// Export the current project to a zip file. `target` is
+    /// `None` for the default filename
+    /// (`YYYYMMDD-<projectname>-export-NN.zip`) under the
+    /// active data root, or `Some(path)` for an explicit
+    /// target. Relative paths resolve under
+    /// `<data-root>/`. Per ADR-0015 §11 the zip excludes
+    /// `playground.db` and `history.log`.
+    Export {
+        target: Option<String>,
+        source: String,
+    },
+    /// Import a previously-exported zip and switch to the
+    /// resulting project. `zip_path` is the user-typed path to
+    /// the source archive (relative to CWD or absolute).
+    /// `as_target` is the optional user-supplied destination
+    /// name; when `None`, the destination is derived from the
+    /// zip's top-level folder. Collisions auto-suffix `-NN`
+    /// (ADR-0015 §11 amendment).
+    Import {
+        zip_path: String,
+        as_target: Option<String>,
+        source: String,
+    },
 }