chore(server): ship the 2.20 standardOverrides drop as a dormant command (#22448)

Follow-up to #22417, per [this
thread](https://github.com/twentyhq/twenty/pull/22417#discussion_r3512187719):
migrate the `2-20/README.md` placeholder into a real command using the
`TWENTY_NEXT_VERSIONS` mechanism.

### What

- Add `DropMetadataStandardOverridesColumnFastInstanceCommand`,
registered against `2.20.0`. It boots (`2.20.0` is in
`TWENTY_ALL_VERSIONS`) but stays **dormant** — the upgrade sequence only
runs `TWENTY_CROSS_UPGRADE_SUPPORTED_VERSIONS` (previous + current), so
it never executes during the 2.19 deploy and activates automatically
when `nx version:bump` promotes 2.20 to current.
- Name constant + unit test (SQL parity, registration against `2.20.0`,
name-constant parity).
- Register it in `instance-commands.constant.ts`.
- Update the `standardOverrides` `@deprecated` comments on object/field
metadata to point at the shipped command.
- Delete `2-20/README.md`.
- Document the "ship a command for a future version" flow in
`docs/UPGRADE_COMMANDS.md` and `.cursor/rules/server-migrations.mdc`
(the mechanism was previously undocumented).

### Note / correction to the README's plan

The old README implied both the command **and** `@WasRemovedInUpgrade`
could be added at 2.20 time. Only the command can ship now: the
decorator's validator runs against the active sequence, so referencing a
still-dormant 2.20 step fails boot with `unknown-step-name`. So the
entity keeps its `WasRemovedInUpgrade<T>` type wrapper for now; the
decorator gets wired (one line, via the name constant) once 2.20 is
current — same deferred-drop shape as `isUIReadOnly`.

### Verification

Could not run `jest`/`typecheck`/`lint` in this environment: `yarn
install` is blocked by egress policy on a git-based transitive dep
(`github.com/electron/node-gyp.git`). Verified by review against the
sibling 2-19 add-column and 2-12 drop commands. **Please let CI run
before merge.**

https://claude.ai/code/session_01KMArJvdEmsX3eAmJLbS1b6

---
_Generated by [Claude
Code](https://claude.ai/code/session_01KMArJvdEmsX3eAmJLbS1b6)_

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packages/twenty-server/src/engine/metadata-modules/object-metadata/object-metadata.entity.ts

@@ -72,8 +72,10 @@ export class ObjectMetadataEntity
   overrides: JsonbProperty<ObjectMetadataOverrides> | null;
 
   /**
-   * @deprecated Superseded by `overrides`; kept readable for pods on the
-   * previous release during a rolling deploy. Drop deferred to 2-20/README.md.
+   * @deprecated Superseded by `overrides`; kept readable for previous-release
+   * pods during a rolling deploy. Dropped by the dormant
+   * DropMetadataStandardOverridesColumnFastInstanceCommand when 2.20 is current;
+   * add @WasRemovedInUpgrade then (its validator rejects a dormant 2.20 step).
    */
   @Column({ type: 'jsonb', nullable: true })
   standardOverrides: WasRemovedInUpgrade<JsonbProperty<ObjectMetadataOverrides> | null>;