feat(nix): shared-state permission model for interactive CLI users (#6796)

* feat(nix): shared-state permission model for interactive CLI users

Enable interactive CLI users in the hermes group to share full
read-write state (sessions, memories, logs, cron) with the gateway
service via a setgid + group-writable permission model.

Changes:

nix/nixosModules.nix:
- Directories use setgid 2770 (was 0750) so new files inherit the
  hermes group. home/ stays 0750 (no interactive write needed).
- Activation script creates HERMES_HOME subdirs (cron, sessions, logs,
  memories) — previously Python created them but managed mode now skips
  mkdir.
- Activation migrates existing runtime files to group-writable (chmod
  g+rw). Nix-managed files (config.yaml, .env, .managed) stay 0640/0644.
- Gateway systemd unit gets UMask=0007 so files it creates are 0660.

hermes_cli/config.py:
- ensure_hermes_home() splits into managed/unmanaged paths. Managed mode
  verifies dirs exist (raises RuntimeError if not) instead of creating
  them. Scoped umask(0o007) ensures SOUL.md is created as 0660.

hermes_logging.py:
- _ManagedRotatingFileHandler subclass applies chmod 0660 after log
  rotation in managed mode. RotatingFileHandler.doRollover() creates new
  files via open() which uses the process umask (0022 → 0644), not the
  scoped umask from ensure_hermes_home().

Verified with a 13-subtest NixOS VM integration test covering setgid,
interactive writes, file ownership, migration, and gateway coexistence.

Refs: #6044

* Fix managed log file mode on initial open

Co-authored-by: Siddharth Balyan <alt-glitch@users.noreply.github.com>

* refactor: simplify managed file handler and merge activation loops

- Cache is_managed() result in handler __init__ instead of lazy-importing
  on every _open()/_chmod_if_managed() call. Avoids repeated stat+env
  checks on log rotation.
- Merge two for-loops over the same subdir list in activation script
  into a single loop (mkdir + chown + chmod + find in one pass).

---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Siddharth Balyan <alt-glitch@users.noreply.github.com>

nix/nixosModules.nix

@@ -560,10 +560,14 @@
       # ── Directories ───────────────────────────────────────────────────
       {
         systemd.tmpfiles.rules = [
-          "d ${cfg.stateDir}                0750 ${cfg.user} ${cfg.group} - -"
-          "d ${cfg.stateDir}/.hermes        0750 ${cfg.user} ${cfg.group} - -"
+          "d ${cfg.stateDir}                2770 ${cfg.user} ${cfg.group} - -"
+          "d ${cfg.stateDir}/.hermes        2770 ${cfg.user} ${cfg.group} - -"
+          "d ${cfg.stateDir}/.hermes/cron   2770 ${cfg.user} ${cfg.group} - -"
+          "d ${cfg.stateDir}/.hermes/sessions 2770 ${cfg.user} ${cfg.group} - -"
+          "d ${cfg.stateDir}/.hermes/logs   2770 ${cfg.user} ${cfg.group} - -"
+          "d ${cfg.stateDir}/.hermes/memories 2770 ${cfg.user} ${cfg.group} - -"
           "d ${cfg.stateDir}/home           0750 ${cfg.user} ${cfg.group} - -"
-          "d ${cfg.workingDirectory}         0750 ${cfg.user} ${cfg.group} - -"
+          "d ${cfg.workingDirectory}         2770 ${cfg.user} ${cfg.group} - -"
         ];
       }
 
@@ -575,7 +579,21 @@
           mkdir -p ${cfg.stateDir}/home
           mkdir -p ${cfg.workingDirectory}
           chown ${cfg.user}:${cfg.group} ${cfg.stateDir} ${cfg.stateDir}/.hermes ${cfg.stateDir}/home ${cfg.workingDirectory}
-          chmod 0750 ${cfg.stateDir} ${cfg.stateDir}/.hermes ${cfg.stateDir}/home ${cfg.workingDirectory}
+          chmod 2770 ${cfg.stateDir} ${cfg.stateDir}/.hermes ${cfg.workingDirectory}
+          chmod 0750 ${cfg.stateDir}/home
+
+          # Create subdirs, set setgid + group-writable, migrate existing files.
+          # Nix-managed files (config.yaml, .env, .managed) stay 0640/0644.
+          find ${cfg.stateDir}/.hermes -maxdepth 1 \
+            \( -name "*.db" -o -name "*.db-wal" -o -name "*.db-shm" -o -name "SOUL.md" \) \
+            -exec chmod g+rw {} + 2>/dev/null || true
+          for _subdir in cron sessions logs memories; do
+            mkdir -p "${cfg.stateDir}/.hermes/$_subdir"
+            chown ${cfg.user}:${cfg.group} "${cfg.stateDir}/.hermes/$_subdir"
+            chmod 2770 "${cfg.stateDir}/.hermes/$_subdir"
+            find "${cfg.stateDir}/.hermes/$_subdir" -type f \
+              -exec chmod g+rw {} + 2>/dev/null || true
+          done
 
           # Merge Nix settings into existing config.yaml.
           # Preserves user-added keys (skills, streaming, etc.); Nix keys win.
@@ -662,6 +680,10 @@ HERMES_NIX_ENV_EOF
             Restart = cfg.restart;
             RestartSec = cfg.restartSec;
 
+            # Shared-state: files created by the gateway should be group-writable
+            # so interactive users in the hermes group can read/write them.
+            UMask = "0007";
+
             # Hardening
             NoNewPrivileges = true;
             ProtectSystem = "strict";