fix(issue-12,8,7): keep broker password out of records, keep fresh install stamps, pack sibling workspaces

Issue #12 - MQTT_PASSWORD was copied into every job record, which any
sub-agent can read. BrokerConfig.to_registry_block no longer persists it;
broker_config_from_job layers the password back from the environment
(.mam.env) at connect time, so publish_event still connects from the record.
_atomic_write_record scrubs broker.password from any record it writes, and
`registry.py scrub-secrets [--dry-run]` migrates an existing registry.
auth_token/dispatch_token stay: the agent's own publish_event needs them.

Issue #8 - update.sh restored the pre-update asset_hashes.txt and version.txt
over the installer's fresh ones, so version.txt named the previous commit and
remove.sh backed up every pristine updated file as "locally modified". The
success path now keeps the installer's stamps and only merges install_state
keys the re-install did not write (gitignore_created); the failure path
still restores everything verbatim.

Issue #7 - the shim's new-session took the first same-cwd pane, always found
the full first workspace, judged it overflow and opened another workspace per
agent (4 + 1 + 1). It now walks every same-cwd workspace in pane order and
takes the first with an idle pane or room in its 2xK grid; a workspace is
created only when all are full (4 + 2).

Tests: test_issue7_workspace_packing.py (3; the mock herdr gains an opt-in
per-workspace 2x2 geometry), test_issue8_update_state.py (1, real
install+update cycle), test_issue12_registry_secrets.py (4); registry CLI
snapshot and help capture refreshed for the intentional scrub-secrets
subcommand; registry.md documents the broker block.

Refs #12 #8 #7

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCgu9Fa9iT7TbJUSzdq4Ff

deploy/update.sh

@@ -229,11 +229,23 @@ if [ $HAS_MAM -eq 1 ]; then
       mkdir -p .mam/delegate_job_logs
       cp -rf .mam.update-tmp/delegate_job_logs/* .mam/delegate_job_logs/
     fi
-    for st in install_state asset_hashes.txt version.txt; do
-      if [ -f ".mam.update-tmp/$st" ]; then
-        cp -f ".mam.update-tmp/$st" .mam/
-      fi
-    done
+    # Issue #8: asset_hashes.txt and version.txt describe the tree the
+    # installer just wrote, so the pre-update copies must NOT come back here -
+    # restoring them made version.txt name the previous commit and remove.sh
+    # treat every updated file as locally modified (spurious backups). Only
+    # install_state carries history worth keeping: merge in any key the fresh
+    # install did not write (e.g. gitignore_created from the original install).
+    # The failure path (restore_on_failure) still puts all three back verbatim.
+    if [ -f ".mam.update-tmp/install_state" ]; then
+      touch .mam/install_state
+      while IFS= read -r line || [ -n "$line" ]; do
+        key="${line%%=*}"
+        [ -n "$key" ] || continue
+        if ! grep -q "^${key}=" .mam/install_state 2>/dev/null; then
+          printf '%s\n' "$line" >> .mam/install_state
+        fi
+      done < .mam.update-tmp/install_state
+    fi
     if [ -d ".mam.update-tmp/skill-backups" ]; then
       mkdir -p .mam/skill-backups
       cp -rf .mam.update-tmp/skill-backups/* .mam/skill-backups/ 2>/dev/null || true