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/database/commands/upgrade-version-command/instance-commands.constant.ts

@@ -86,6 +86,7 @@ import { EncryptNonSecretApplicationVariableSlowInstanceCommand } from 'src/data
 import { MigrateAiModelPreferencesSlowInstanceCommand } from 'src/database/commands/upgrade-version-command/2-9/2-9-instance-command-slow-1799000010000-migrate-ai-model-preferences';
 import { AddFolderImportToMessageFolderPendingSyncActionFastInstanceCommand } from './2-15/2-15-instance-command-fast-1781714499016-add-folder-import-to-message-folder-pending-sync-action';
 import { AddViewKanbanColumnWidthFastInstanceCommand } from './2-15/2-15-instance-command-fast-1781900000000-add-view-kanban-column-width';
+import { DropMetadataStandardOverridesColumnFastInstanceCommand } from './2-20/2-20-instance-command-fast-1825000000000-drop-metadata-standard-overrides-column';
 
 export const INSTANCE_COMMANDS = [
   AddViewFieldGroupIdIndexOnViewFieldFastInstanceCommand,
@@ -174,4 +175,5 @@ export const INSTANCE_COMMANDS = [
   BackfillTsVectorFieldMetadataIdOnSearchFieldMetadataSlowInstanceCommand,
   AddMetadataOverridesColumnFastInstanceCommand,
   BackfillMetadataOverridesSlowInstanceCommand,
+  DropMetadataStandardOverridesColumnFastInstanceCommand,
 ];