docs: reconcile Disposable claim with shipped core () => void

[thorwhalen]

Three design docs claimed every `register*` returns a `Disposable` (`{ dispose(): void }`), but shipped core (`packages/core/src/registry.ts`) returns a bare `() => void` unregister thunk:

• `register(cmd): () => void`
• `registerAll(cmds): () => void` — the returned thunk unregisters the whole group atomically (the owner-scoped unload primitive)
• `onCommandsChanged(listener): () => void`

This sides with the implementation (the intended design — docs/hand-written-sandbox.md's "Faithfulness note" already documents the `() => void` reality and the deliberate divergence from research-9's idealized interfaces). Changing core to return a `Disposable` would be a breaking public-API change for no functional gain.

Changed (3 files)

• `docs/redesign_takeaways.md` §1.5 — heading + body
• `docs/v1_plan.md` — Owner-scoped lifecycle table row
• `docs/reference_notes.md` — the acture-directed "Dos" lesson in the VS Code ref_11 note

Left intact (intentional)

• The VS Code factual line in `reference_notes.md` (`registerCommand` returns a `Disposable` — correct about VS Code).
• `docs/hand-written-sandbox.md` (already correct), the research-9 design artifact, and `wrapex_carryover.md`.
• Other packages' legitimate `.dispose()` methods (`acture-undo`, `acture-sandbox` runner, telemetry/devtools).

Verified by a 3-lens adversarial pass (API fidelity vs shipped core / residual-claim sweep / internal-consistency) — all clean.

Closes #39