feat(dashboard): Enter on agents pane re-attaches to bottle

PRD 0020 chunk 3. Enter on a focused agents-pane row drops to a
claude session inside the selected bottle. Works for both
dashboard-owned bottles (looks up the stored Bottle handle in
the main loop's `bottles` dict) and externally-discovered ones
(synthesizes a DockerBottle from the slug → `claude-bottle-<slug>`
container name).

For the synthesized path, the `--append-system-prompt-file`
target resolves via metadata.json + the manifest's agent prompt
if both can be read; otherwise the re-attach runs without the
flag (claude defaults to no system prompt, the bottle's other
state is untouched).

Shares the curses.endwin → attach → refresh handoff with the
chunk-2 new-agent flow via a new `_attach_to_bottle` helper.
Footer reshuffled to advertise `[Enter] view/attach`. 464 unit
tests pass (3 new for `_bottle_for_slug`).

tests/unit/test_dashboard_active_agents.py

@@ -357,6 +357,28 @@ class TestSelectedAgent(unittest.TestCase):
         )
 
 
+class TestBottleForSlug(unittest.TestCase):
+    """Re-attach target resolution (PRD 0020 chunk 3). Dashboard-
+    owned bottles return the stored handle as-is; non-owned bottles
+    get a synthesized DockerBottle backed by the slug-derived
+    container name."""
+
+    def test_owned_bottle_returns_held_handle(self):
+        sentinel = object()
+        bottles = {"dev-abc": (None, sentinel, "dev-abc")}
+        bottle, _ = dashboard._bottle_for_slug("dev-abc", bottles, None)
+        self.assertIs(sentinel, bottle)
+
+    def test_unowned_synthesizes_docker_bottle(self):
+        bottle, _ = dashboard._bottle_for_slug("dev-xyz", {}, None)
+        # The synth wraps the slug-derived container name.
+        self.assertEqual("claude-bottle-dev-xyz", bottle.name)
+
+    def test_unowned_without_manifest_omits_prompt_path(self):
+        bottle, hint = dashboard._bottle_for_slug("dev-xyz", {}, None)
+        self.assertEqual("", hint)
+
+
 class TestOperatorEditFlowGuards(_FakeHomeMixin, unittest.TestCase):
     """Chunk-4 contract: the edit flow refuses when the selected
     agent doesn't have the required sidecar running. The discover-