fix(auth): make snapshot-and-clear one atomic storage step (#2144)

Copilot review round 18. Three of its findings were the same check-then-act:
`planOAuthRevocation` is asynchronous, so the gap between its reads and the
following `storage.clear()` is still a window — shorter than the network one
closed in round 13, but the same shape. An OAuth completion landing at any of
those awaits saved a grant the clear then destroyed.

`OAuthStorage.takeRevocationSnapshot(serverUrl)` reads what revocation needs
and clears the server in ONE synchronous pass over the in-memory state, with
the persist after the mutation. `clearAndPlanRevocation` replaces
`planOAuthRevocation`, and there is no separate `clear` call left in any
caller — the clear cannot be forgotten or reordered.

The snapshot returns values unparsed, deliberately: schema validation is pure
and belongs after the mutation, since running it inside would reintroduce the
`await` this exists to remove.

Two consequences worth naming. A failure of the take-and-clear now REJECTS
rather than returning an outcome, because the state was not cleared and the
caller's contract is that it was; both clients already have a rejection path.
And cross-process atomicity is explicitly not claimed — nothing in this store
takes a lock, every mutation is a read-modify-write over a loaded snapshot, and
`clear` alone always had that property.

Also from round 18:

- The CLI's budget check ran before considering `plan.outcome`, so a key that
  needed no network was reported as budget-exhausted — warning that a grant may
  be live when that key held none.
- A disconnect failure after a successful clear was reported as "could not
  clear OAuth state" in both clients. It now goes to the disconnect channel,
  and the success is still reported.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0177mPHAdECD18nLLCTwR5rh
Signed-off-by: cliffhall <cliff@futurescale.com>

core/auth/index.ts

@@ -140,8 +140,8 @@ export {
   aggregateOutcomes,
   buildRevocationRequest,
   revocationAuthMethods,
+  clearAndPlanRevocation,
   executeOAuthRevocation,
-  planOAuthRevocation,
   revokeToken,
   selectRevocableToken,
 } from "./revocation.js";