Add GH_OST_INSTANT_DDL for gh-ost-on-success hook

[meiji163]

Description

This PR adds boolean environment variable GH_OST_INSTANT_DDL for gh-ost-on-success hook, so one can tell whether gh-ost used instant DDL successfully

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[Copilot]

Pull request overview

Adds a new boolean environment variable (GH_OST_INSTANT_DDL) to the gh-ost-on-success hook so hook scripts can determine whether the migration completed via Instant DDL.

Changes:

• Extend HooksExecutor.onSuccess() to accept an instantDDL boolean and export it as GH_OST_INSTANT_DDL to success hooks.
• Wire the new onSuccess(true/false) calls from the migrator’s success paths (instant vs non-instant).
• Document GH_OST_INSTANT_DDL in the hooks documentation.

Show a summary per file

File	Description
go/logic/migrator.go	Passes instantDDL into the success hook and adds a noop/instant-ddl early return (currently problematic).
go/logic/hooks.go	Adds GH_OST_INSTANT_DDL env var injection for gh-ost-on-success hooks.
doc/hooks.md	Documents the new GH_OST_INSTANT_DDL variable for the success hook.

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• Files reviewed: 3/3 changed files
• Comments generated: 3