fix(core): make hard delete owner-only in ScopedStack

[cuibonobo]

Summary

Closes the first, unblocked work item from #59.

ScopedStack.delete() called requireDeletable() (checks the record-level write bit or a delete-own/delete-any grant) and then forwarded opts straight to the adapter — so a write: true holder, or a delete-any grant holder, could pass { hard: true } and permanently destroy the record and its version history. There was no test exercising this path.

Per #59's stated trust model — "anything a write-holder does, the owner can undo" — hard delete has no undo, so it doesn't belong behind any delegated-access mechanism. This PR makes hard delete owner-only:

• ScopedStack.delete() now throws StackPermissionError when a non-owner passes { hard: true }, regardless of whether they'd otherwise be authorized to delete (write bit or grant).
• Non-owners remain able to soft-delete, which stays recoverable.
• Spec note added under Deletion stating the owner-only restriction and that it applies only to the asEntity()/ScopedStack wrapper (plain Stack is unscoped and trusted-by-definition per Single-writer storage model: durability fixes for adapter-local + spec/README guidance on multi-app topology #45).

Out of scope (tracked separately)

#59 has other work items — an undelete API, versioned/audited association changes, and the full "recoverability trust model" spec writeup — that are genuine prerequisites tracked as their own open issues (#60, #61) and don't exist in code yet. This PR only lands the carve-out that #59 itself flags as having no dependencies.

Test plan

• Added tests: write-bit holder can soft-delete but not hard-delete; owner can hard-delete; delete-any grant holder cannot hard-delete
• pnpm --filter @haverstack/core test — 192 tests passing
• pnpm --filter @haverstack/core typecheck — clean
• pnpm --filter @haverstack/core lint — clean
• prettier --check on changed files — clean

Refs #59, #60, #61, #45

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