feat(dashboard): agent-scoped e/p, drop discover-and-prompt path

PRD 0019 chunk 4 (final). The `e` (routes edit) and `p` (pipelock
edit) keys now require an agent selection in the agents pane.
Pressing them with the proposals pane focused, with no active
agents, or with an out-of-range selection is a no-op with a
status hint ("no agent selected; Tab into the agents pane first").

The discover-and-prompt scaffolding inside
`_operator_edit_routes_flow` / `_operator_edit_allowlist_flow` /
`_operator_edit_flow` is gone. The flows now take an `ActiveAgent`
+ required-service name; they refuse with a clear message when
the bottle lacks the requested sidecar (e.g., `routes edit`
against a bottle with no `bottle.egress.routes` declared). The
`discover_egress_slugs` + `discover_pipelock_slugs` +
`_discover_active_with_service` helpers come out — they had no
remaining callers.

Footer now reads `[e/p] edit selected agent`.

tests/unit/test_dashboard.py

@@ -490,24 +490,6 @@ class TestOperatorEditRoutes(_FakeHomeMixin, unittest.TestCase):
         self.assertEqual([], read_audit_entries("egress", "dev"))
 
 
-class TestDiscoverEgressSlugs(unittest.TestCase):
-    """Slug-extraction parsing — exercises only the parsing path; the
-    docker ps invocation itself is environment-dependent (and tested
-    implicitly by the integration test)."""
-
-    def test_returns_empty_when_docker_unavailable(self):
-        # Force a failure by setting PATH to a dir with no docker
-        # binary. The discover helper swallows the non-zero rc.
-        import os
-        original = os.environ.get("PATH", "")
-        os.environ["PATH"] = "/nonexistent-no-docker-here"
-        try:
-            self.assertEqual([], dashboard.discover_egress_slugs())
-            self.assertEqual([], dashboard.discover_pipelock_slugs())
-        finally:
-            os.environ["PATH"] = original
-
-
 class TestOperatorEditAllowlist(_FakeHomeMixin, unittest.TestCase):
     """PRD 0015 Phase 3: operator-initiated pipelock allowlist edit."""