feat: secrets join + init --remote + verified onboarding (EGB-671)

Add second-machine onboarding as a first-class verb rather than a manual
clone + key-copy sequence:

- secrets join --remote <url> --key <path>: clone the vault, install the key
  at mode 600, then decrypt-test it before declaring success. An empty vault
  reports "nothing to verify yet" (never a false VERIFIED); a wrong key fails
  loudly. Reuses the audited core (resolve_store, get_pubkey, _verify_all) —
  no security logic re-implemented.
- secrets init --remote <url>: wire the remote and push the initial store so
  the upstream branch exists (fixes the commit_and_push_secrets pull --ff-only
  die against a brand-new empty remote). init also offers an interactive
  first-add of a project (default No; skipped under --yes / non-interactive).
- cmd_push first-manifest scaffold writes an explicit committed options.autoAdd
  value, asked once when interactive (EGB-677 contract #2).
- secrets pull now dies loudly when a blob fails to decrypt (all three decrypt
  paths) instead of warning and exiting 0 — a wrong key can't pass silently.
- Interactive prompts gate on stdin AND stdout being ttys, so bats/CI never hang.
- Dispatcher routes init/join args correctly; second-machine trap points at join.

Tests: 20 new (join, autoAdd, pty-no-hang regression); 2 trap tests updated.

test/manifest.bats

@@ -167,6 +167,17 @@ load test_helper
   [ "$output" = "2" ]
 }
 
+@test "bootstrap: first push writes an explicit options.autoAdd value (EGB-677 contract #2)" {
+  init_with_remote
+  create_project_dir autoaddproj
+  # Non-interactive (bats has no tty): the prompt is skipped and the tool
+  # default (ON) is written explicitly so the value is committed + team-shared.
+  run "$SECRETS_BIN" push
+  [ "$status" -eq 0 ]
+  run jq -r '.options.autoAdd' .secrets.json
+  [ "$output" = "true" ]
+}
+
 @test "failed push leaves no bootstrap manifest behind" {
   init_with_remote
   mkdir -p "$WORK_DIR/emptyproj"