fetch the agent's backend env file from the store, seeded from the existing file

the agent's own root unit reads swarm/agents/<agent>/backend-env into /run and
creates it from backendEnvironmentFile with -cas=0 when the store holds nothing.
the shared agent policy gains create (not update) on that one leaf. the harness
and the subagent daemon load the old file first and the fetched one after it.

nix/script-tests/agent-backend-env-fake-bao.sh

@@ -0,0 +1,77 @@
+#!/usr/bin/env bash
+# The stub `bao` for ./agent-backend-env.nix. Leaves are files under
+# `$FAKE_BAO_KV`, one per path. Every call is logged, a `kv` call without the
+# token `login` printed fails, and the answers are the ones openbao gives:
+# `kv get` of a leaf nobody wrote exits 2 with "No value found at <path>", and
+# a policy granting `create` and `read` only refuses a second `-cas=0` write
+# with a 403.
+#
+# `FAKE_BAO_LOGIN_RC` makes the login fail, `FAKE_BAO_DENY_READ` and
+# `FAKE_BAO_DENY_WRITE` answer 403 as a policy that grants neither does, and
+# `FAKE_BAO_RACE` has another writer create the leaf with that text between
+# this run's first read and its write.
+echo "$*" >>"$FAKE_BAO_LOG"
+case "$1 $2" in
+  "login -method=cert")
+    [ "${FAKE_BAO_LOGIN_RC-0}" = 0 ] || {
+      echo 'Error authenticating: Put "https://bao.t.local:8200/v1/auth/cert/login": dial tcp: lookup bao.t.local: no such host' >&2
+      exit 2
+    }
+    printf 's.fake'
+    ;;
+  "kv get" | "kv put")
+    [ "${BAO_TOKEN-}" = s.fake ] || {
+      echo "fake bao: kv called without the login's token" >&2
+      exit 97
+    }
+    ;;&
+  "kv get")
+    [ "$3" = -field=env ] || {
+      echo "fake bao: get without -field=env: $*" >&2
+      exit 95
+    }
+    leaf=$FAKE_BAO_KV/$(printf '%s' "$4" | tr / _)
+    [ -z "${FAKE_BAO_DENY_READ-}" ] || {
+      printf 'Error reading %s: Error making API request.\n\nURL: GET https://bao.t.local:8200/v1/%s\nCode: 403. Errors:\n\n* 1 error occurred:\n\t* permission denied\n' "$4" "$4" >&2
+      exit 2
+    }
+    if [ -n "${FAKE_BAO_RACE-}" ] && [ ! -e "$FAKE_BAO_KV/.raced" ]; then
+      : >"$FAKE_BAO_KV/.raced"
+      printf '%s' "$FAKE_BAO_RACE" >"$leaf"
+      echo "No value found at ${4/secret\//secret/data/}" >&2
+      exit 2
+    fi
+    if [ ! -e "$leaf" ]; then
+      echo "No value found at ${4/secret\//secret/data/}" >&2
+      exit 2
+    fi
+    if [ "$(cat "$leaf.field" 2>/dev/null)" = other ]; then
+      echo 'Field "env" not present in secret' >&2
+      exit 1
+    fi
+    cat "$leaf"
+    ;;
+  "kv put")
+    [ "$3" = -cas=0 ] || {
+      echo "fake bao: put without -cas=0: $*" >&2
+      exit 96
+    }
+    leaf=$FAKE_BAO_KV/$(printf '%s' "$4" | tr / _)
+    case "$5" in
+      env=@*) ;;
+      *)
+        echo "fake bao: put of something other than env=@file: $*" >&2
+        exit 94
+        ;;
+    esac
+    if [ -n "${FAKE_BAO_DENY_WRITE-}" ] || [ -e "$leaf" ]; then
+      printf 'Error writing data to %s: Error making API request.\n\nURL: PUT https://bao.t.local:8200/v1/%s\nCode: 403. Errors:\n\n* 1 error occurred:\n\t* permission denied\n' "$4" "$4" >&2
+      exit 2
+    fi
+    cp "${5#env=@}" "$leaf"
+    ;;
+  *)
+    echo "fake bao: unexpected call: $*" >&2
+    exit 98
+    ;;
+esac