Issue for this PR

Closes #15873

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

opencode auth login https://opencode.cloudflare.dev/ and opencode auth login https://opencode.cloudflare.dev store separate entries in auth.json — the URL is used as the cache key verbatim. The trailing-slash variant also produces https://opencode.cloudflare.dev//.well-known/opencode during fetch.

• Centralize trailing-slash stripping in Auth.set and Auth.remove so all callers (CLI, server API) normalize automatically
• Auth.set also deletes any pre-existing trailing-slash variant, cleaning up stale entries from before the fix
• Auth.remove deletes both the raw and normalized key so logout works regardless of form
• Callsite normalization in the CLI login handler and config loader fixes the fetch URL construction (${url}/.well-known/opencode)

How did you verify your code works?

• Direct integration tests for Auth.set/Auth.remove normalization — no mocks, real I/O against a temp auth.json
• Config-level regression test verifying trailing-slash keys produce correct .well-known fetch URLs
• bun run typecheck and existing test suite pass

Screenshots / recordings

N/A — no UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR