[codeandsolder]

Pins and documents the gateway's intentionally asymmetric logout/session behavior instead of leaving it implicit.

Changes:

• expand the existing gateway logout E2E into a two-login regression
• prove /auth/logout flushes the exact cookie session used for logout
• prove another cookie session for the same subject remains authenticated
• prove refresh-token revocation is subject-wide, so refresh credentials from both logins are rejected after one /auth/logout
• document that already-issued access JWTs remain valid until exp without a separate revocation mechanism
• distinguish /session/logout (current cookie session only) from /auth/logout (subject-wide refresh revocation + current cookie session)
• document tower_sessions::MemoryStore as a deliberately process-local demo backend: restarts invalidate sessions and replicas do not share them
• update handler/session docs, README, and canonical SKILL.md together

The temporary write-capable helper workflow used for exact large-file edits was removed before this PR was opened.

Kept draft until the normal full CI matrix validates the final tree. Intentionally not auto-merged.