Only check for pending migrations at login

omarchy-update-user-notify.path watched /usr/share/omarchy/migrations, but
pacman writes that directory during every update, including the blessed
omarchy update, which runs omarchy-migrate a step later. The watcher fired a
critical notification for the migrations the update was already applying in
the visible terminal. A watcher cannot tell that apart from a bypassed
pacman -Syu, so the only trigger that never collides with a running update is
a once-per-login check.

The service that already ran at graphical-session.target is now the whole
mechanism, renamed after the command it runs. That is also all the second-user
case needs: markers are per-user, so anyone who did not run the update finds
them missing at their next login.

Login timing means the toast can be sent before the shell has claimed
org.freedesktop.Notifications, so the notifier waits for a live server first.
The wait is omarchy-first-run's, lifted into omarchy-notification-wait rather
than duplicated.

The package keeps omarchy-update-user-notify.service as a symlink onto the new
unit. Existing users hold an absolute wants symlink to the old path, and the
migration that repoints it only runs for users who run an update, which is the
opposite of who the notifier is for.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/update-process.md

@@ -149,20 +149,37 @@ High-level flow:
 sudo pacman -Syu
   ├─ pre-transaction guard aborts and tells the user to run omarchy update
   └─ if explicitly bypassed, upgrades omarchy and related packages
-  └─ user session notices migration directory changes
-       ├─ omarchy-update-user-notify.path triggers, if enabled
+  └─ at that user's next login
+       ├─ omarchy-migrate-notify.service starts with graphical-session.target
        ├─ omarchy-migrate-notify checks omarchy-migrate --pending
        ├─ if this user has missing migration state, show notification
        └─ click opens terminal: omarchy-migrate
 ```
 
+Login is deliberately the only trigger. A watcher on the packaged migration
+directory cannot distinguish a bypassed `pacman -Syu` from the package
+transaction inside a normal `omarchy update`, so it fired notifications for
+migrations that `omarchy-migrate` was about to apply in the visible update
+terminal. The retired unit was `omarchy-update-user-notify.path`.
+
 Fallbacks:
 
-- `omarchy-first-run` enables the user notification path unit.
-- `omarchy-first-run` also invokes `omarchy-migrate-notify` on graphical
-  startup, so users who updated before the path unit existed still get prompted
-  if they have missing migration state.
+- `omarchy-first-run` enables `omarchy-migrate-notify.service`, which also
+  covers users created after install: their per-user migration markers are
+  missing, so their first login prompts them to run every shipped migration.
+- The package ships `omarchy-update-user-notify.service` as a symlink onto
+  `omarchy-migrate-notify.service`. Users set up before the rename hold an
+  absolute `graphical-session.target.wants` symlink to the old path, and the
+  migration that repoints it only runs for users who run an update — the
+  opposite of who the notifier is for. The alias can be dropped once installs
+  have run migration `1785095882`.
+- The notifier waits for a live notification server before sending, because
+  `graphical-session.target` can be reached before the shell claims
+  `org.freedesktop.Notifications`.
 - The notifier is only a prompt. It does not run migrations in the background.
+- A session that is already open when another user updates is not re-checked;
+  it picks the migrations up at its next login, or whenever that user runs
+  `omarchy-migrate` or `omarchy update`.
 - Direct pacman updates do not run `omarchy-hook post-update` unless the user
   explicitly runs that hook; without a package-update marker, the only pending
   state we can derive is missing per-user migration markers.
@@ -209,7 +226,7 @@ scripts.
 | `omarchy-update-system-pkgs` | Runs `sudo env OMARCHY_UPDATE_PACMAN=1 pacman -Syu --noconfirm` with targeted transition `--overwrite` entries so the ALPM guard allows the transaction and early package-layout conflicts are handled. | **Keep for now.** Small leaf command, clear/testable. |
 | `omarchy-migrate` | Public migration command. Waits for pacman, then runs all pending migrations for the current user. Supports `--pending`. | **Keep.** This replaces the discarded `omarchy-update-user-finalize` name and no longer needs `--force`. |
 | `omarchy-update-pacman-guard` | ALPM pre-transaction guard that aborts direct `pacman -Syu` style upgrades unless Omarchy set `OMARCHY_UPDATE_PACMAN=1` or the user explicitly set `OMARCHY_ALLOW_DIRECT_PACMAN=1`. | **Keep internal/hidden.** This is what nudges users back to `omarchy update`. |
-| `omarchy-migrate-notify` | Internal notification helper for direct pacman updates. Uses `omarchy-migrate --pending` and shows notification only when this user has pending migrations. | **Keep internal/hidden.** Clear name now that the public command is `omarchy-migrate`. |
+| `omarchy-migrate-notify` | Internal login-time notification helper. Uses `omarchy-migrate --pending` and shows a notification only when this user has pending migrations. | **Keep internal/hidden.** Clear name now that the public command is `omarchy-migrate`. |
 | `omarchy-update-user-notify` | Hidden compatibility wrapper for `omarchy-migrate-notify`. | **Temporary.** Keep only for old callers. |
 | `omarchy-update-available` | Update checker for shell widget and post-update refresh. | **Keep.** Could eventually be renamed `omarchy-update-check`, but current name matches widget semantics. |
 | `omarchy-update-aur-pkgs` | Updates AUR packages with `yay -Sua` if foreign packages exist and AUR is reachable. | **Question.** Omarchy is package-backed now, but users may still install AUR packages. Keep for now. |
@@ -229,7 +246,8 @@ scripts.
      idempotent when they repair machine-wide state.
 
 2. **Migration notification naming**
-   - The real helper is `omarchy-migrate-notify`.
+   - The real helper is `omarchy-migrate-notify`, started by
+     `omarchy-migrate-notify.service`.
    - `omarchy-update-user-notify` remains only as a hidden compatibility wrapper.
 
 3. **Update pipeline ownership**