fix(fleet): INI fully replaces prior decky state on redeploy

Submitting an INI with a single [decky1] was silently redeploying the
deckies from the *previous* deploy too. POST /deckies/deploy merged the
new INI into the stored DecnetConfig by name, so a 1-decky INI on top of
a prior 3-decky run still pushed 3 deckies to the worker. Those stale
decky2/decky3 kept their old IPs, collided on the parent NIC, and the
agent failed with 'Address already in use' — the deploy the operator
never asked for.

The INI is the source of truth for which deckies exist this deploy.
Full replace: config.deckies = list(new_decky_configs). Operators who
want to add more deckies should list them all in the INI.

Update the deploy-limit test to reflect the new replace semantics, and
add a regression test asserting prior state is dropped.

tests/api/config/test_deploy_limit.py

@@ -19,11 +19,16 @@ def mock_network():
 
 @pytest.mark.anyio
 async def test_deploy_respects_limit(client, auth_token, mock_state_file):
-    """Deploy should reject if total deckies would exceed limit."""
+    """Deploy should reject if the *submitted* INI exceeds the limit.
+    The INI is the source of truth — prior state is fully replaced — so the
+    check runs on the new decky count alone."""
     await repo.set_state("config_limits", {"deployment_limit": 1})
     await repo.set_state("deployment", mock_state_file)
 
-    ini = """[decky-new]
+    ini = """[decky-a]
+services = ssh
+
+[decky-b]
 services = ssh
 """
     resp = await client.post(
@@ -31,11 +36,33 @@ services = ssh
         json={"ini_content": ini},
         headers={"Authorization": f"Bearer {auth_token}"},
     )
-    # 2 existing + 1 new = 3 > limit of 1
+    # 2 new deckies > limit of 1
     assert resp.status_code == 409
     assert "limit" in resp.json()["detail"].lower()
 
 
+@pytest.mark.anyio
+async def test_deploy_replaces_prior_state(client, auth_token, mock_state_file):
+    """Submitting an INI with 1 decky must not silently re-include the 2
+    deckies from prior state (that caused the 'Address already in use'
+    regression when stale decky2/decky3 redeployed on stale IPs)."""
+    await repo.set_state("config_limits", {"deployment_limit": 10})
+    await repo.set_state("deployment", mock_state_file)
+
+    ini = """[only-decky]
+services = ssh
+"""
+    resp = await client.post(
+        "/api/v1/deckies/deploy",
+        json={"ini_content": ini},
+        headers={"Authorization": f"Bearer {auth_token}"},
+    )
+    assert resp.status_code == 200
+    persisted = await repo.get_state("deployment")
+    names = [d["name"] for d in persisted["config"]["deckies"]]
+    assert names == ["only-decky"]
+
+
 @pytest.mark.anyio
 async def test_deploy_within_limit(client, auth_token, mock_state_file):
     """Deploy should succeed when within limit."""