🤖 fix: follow-ups for default unrelated-messaging consent (#4440)

[ThomasK33]

Non-blocking follow-ups from the final readiness review of #4440 (default unrelated-messaging consent for user-created workspaces):

1. Pending mark leak. If create() takes its removing/aborted branch, or throws between marking a deferred checkout pending and retaining the settlement, the pendingDefaultUnrelatedConsent entry is never cleared. This is harmless (only materializeDeferredCheckout reads it, and IDs are random), but it should be cleared there for tidiness.
2. Failed deferred checkout stays off. A deferred checkout that fails without a sanitization error leaves a usable workspace with consent off, while an immediate checkout is granted. This fails closed. Either grant on that path once sanitization has succeeded, or mention the caveat in the tool and docs text.
3. Multi-project has no plugin-override sanitization at all. This predates 🤖 feat: opt new user-created workspaces in to unrelated messaging by default #4440, but default consent now makes multi-project workspaces reachable by other task trees as soon as they are created. Decide whether multi-project needs registration-time sanitization.
4. Test gap. Add a create()-level deferred-checkout test that asserts the mark is set before the announcement and cleared after settlement. Existing tests call materializeDeferredCheckout directly.

Related: #4444, #4446, #4453.

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[linear-code]

MUX-205

[ThomasK33]

Picked up by Xum workspace workspace-24 (4426dd3ae8) as part of the approved consent-cluster plan (#4455 → #4305 → #4453, sequential PRs).

This issue: PR1 of 3 (fail-closed finalization of the pending mark in create()/fork(), item-4 test; items 2 and 3 closed with evidence).

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[ThomasK33]

Fixed by #4814 (merged as 39e49b2).

1. Pending-mark leak, now durable after 🤖 fix: another backend's consent toggle cancels a pending default consent #4783: fixed. create() and fork() each have one finally that clears the mark on every exit that did not hand it to the deferred checkout's settlement.
2. Failed deferred checkout stays off: kept on purpose (fail closed). The checkout is missing or incomplete, and a comment at the branch now says so. If a user opts in anyway, 🤖 fix: clarify unrelated-message acceptance for missing-checkout recipients #4305's fix (next PR) refuses unrelated sends early while the checkout is missing.
3. Multi-project sanitization: not needed, because the container dir is always fresh (createContainer uses a non-recursive mkdir). The one alias was a project folder named like .xum/.mux (case-insensitively, trailing dots/spaces trimmed), whose link would redirect the container's overrides file. New multi-project workspaces now refuse such a project.
4. create()-level test: added (the mark is set at announcement, then settled into consent).

Out-of-scope residual, tracked in #4818: a create()/fork() that returns Err after registration still keeps the row.

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