*: add read-first DDL framework notes

[wjhuang2016]

What problem does this PR solve?

Issue Number: close #66093

Problem Summary:

• DDL framework knowledge is scattered across code and design docs, and it’s easy to implement DDL behavior ad-hoc in pkg/executor/ without understanding the job-based, owner-driven execution pipeline.
• That increases correctness risk and review overhead, because changes can bypass intended invariants (schema state machine, schema sync, resumability, reorg/backfill).

What changed and how does it work?

• Added a “read-first” DDL framework note set under docs/note/ddl/ with an index + reading order, covering execution flow, job lifecycle, reorg/backfill pointers, a dev checklist, and a file map.
• Strengthened agent guidance in AGENTS.md with DDL module-only rules:
  • read the DDL notes before DDL work,
  • treat notes as hypotheses in debugging (validate via code/tests),
  • update notes immediately when doc/code diverge.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[pantheon-ai]

❌ Request Failed

The system couldn’t send or save your message because it’s missing the information that identifies which user it belongs to. Please try again after signing in or making sure your account is selected.

Please try again or contact support if the issue persists.

_{Learn more about Pantheon AI}

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign zhaoxinyu for approval. For more information see the Code Review Process.
Please ensure that each of them provides their approval before proceeding.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[lance6716]

Very helpful! (will help to comment soon)

[lance6716]

How about removing this line? Error path is less important than other main logic

[wjhuang2016]

I don't verify the whole context for now; it still needs further verification

[lance6716]

I'm not sure if my impression helps 😂

Acts as the SQL abstraction layer that wraps internal distributed DDL module for end-user interaction. Runs on the TiDB node that user connected.

[lance6716]

The entry point for distributed DDL tasks. Acts as a framework client responsible for verification, serialization, and blocking wait. Runs on the TiDB node that user connected.