Archive unify-auth-completion + sync spec delta (task 5.1)

Applies the ADDED requirement from
openspec/changes/unify-auth-completion/specs/authentication-methods/
into openspec/specs/authentication-methods/spec.md:

- Single web auth completion chokepoint (completeAuth at
  apps/journal/app/lib/auth/completion.server.ts) with five scenarios
  covering passkey register/login finish, magic-link verify-code,
  magic-link click-through, and returnTo sanitization.

Path in the synced requirement is .server.ts (the post-rename name),
not the .ts the delta originally captured.

Change moved to openspec/changes/archive/2026-05-08-unify-auth-completion/.
15/15 tasks complete.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

openspec/changes/archive/2026-05-08-unify-auth-completion/tasks.md

@@ -26,5 +26,5 @@
 
 ## 5. Documentation + follow-up
 
-- [ ] 5.1 At archive time, apply the spec delta in `specs/authentication-methods/` to `openspec/specs/`.
+- [x] 5.1 At archive time, apply the spec delta in `specs/authentication-methods/` to `openspec/specs/`.
 - [x] 5.2 Update import paths app-wide from `auth.server.ts` to `./auth/session.ts` and drop the re-exports. (Folded into this PR after the smoke test confirmed everything works.)