docs: add agents guide for contributors

[Justin-ZS]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Add a concise AGENTS.md at the repository root to summarize the repo layout, source-vs-generated files, contributor docs, and common local checks.

Fixed issues

• N.A.

Details

Before: What was the problem?

There was no short repo-specific orientation document for coding agents or contributors, so basic repository context had to be reconstructed from the codebase and supporting docs each time.

After: How does it behave after the fixing?

The repo now includes an AGENTS.md that briefly explains the project layout, the role of export and entry files, which files are generated, which contributor-facing docs to follow, and the main local validation commands.

Document Info

One of the following should be checked.

• This PR doesn't relate to document changes
• The document should be updated later
• The document changes have been made in apache/echarts-doc#xxx

Misc

Security Checking

• This PR uses security-sensitive Web APIs.

ZRender Changes

• This PR depends on ZRender changes (ecomfe/zrender#xxx).

Related test cases or examples to use the new APIs

N.A.

Merging options

• Please squash the commits into a single one when merging.

Other information

Validation not run because this is a documentation-only change.

[echarts-bot]

Thanks for your contribution!
The community will review it ASAP. In the meanwhile, please checkout the coding standard and Wiki about How to make a pull request.

The pull request is marked to be PR: author is committer because you are a committer of this project.

To reviewers: If this PR is going to be described in the changelog in the future release, please make sure this PR has one of the following labels: PR: doc ready, PR: awaiting doc, PR: doc unchanged

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[echarts-bot]

Document changes are required in this PR. Please also make a PR to apache/echarts-doc for document changes and update the issue id in the PR description. When the doc PR is merged, the maintainers will remove the PR: awaiting doc label.

[github-actions]

The changes brought by this PR can be previewed at: https://echarts.apache.org/examples/editor?version=PR-21587@f64ba6b