docs: home the login plan under docs/plans and record the auth decision

[kanushka]

Follow-up to the #17 stack (part of #17).

• Moves docs/superpowers/plans/2026-08-05-login-first-slice.md to docs/plans/login-first-slice.md, matching the existing docs/plans/ naming convention, and removes the docs/superpowers/ directory. The plan's dead reference to a never-committed spec file now points at issue wso2 login first slice: browser PKCE and inline client credentials against Asgardeo and IS #17, and the agent-tooling header is dropped.
• Indexes the plan in docs/README.md alongside the two existing plans.
• Adds ADR 0004: Shell-Brokered Authentication — the durable decisions the login slice made: shell-only credential custody, public client + PKCE, keychain-only sessions with fail-closed refusal, per-kind token sources behind the broker seam, and verification of issued tokens (a deployment that cannot narrow is refused, never granted broader access).

https://claude.ai/code/session_01RojiAgW9hi3b9f9G6ZXBVp

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