feat(agents): add AGENTS.md template

[marnagy]

What this PR does / why we need it

Jira ID

DT-5083

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Rovo Dev code review: Rovo Dev has reviewed this pull request
Any suggestions or improvements have been posted as pull request comments.

[atlassian]

The issue is ready for review, and the below acceptance criteria have been met:

• ✅ Create an AGENTS.md file in the stencil-base module.
• ✅ AGENTS.md file is owned by stencil-base.
• ✅ AGENTS.md includes a block for repo-specific instructions.
• ✅ AGENTS.md includes a module hook that other stencil modules can use to add content.

The below acceptance criteria have not been met:

• ❌ Add AGENTS.md content to stencil-golang using the module hook.
• ❌ Add AGENTS.md content to stencil-graphql using the module hook.
• ❌ Add AGENTS.md content to stencil-outreach using the module hook.

Code Reviewer could not determine whether the following acceptance criteria have been met:

• After the AGENTS.md and module hook changes are released, notify QSS that they can add agents.md content to their modules.

Check Jira issue

[malept]

AGENTS.md should teach AI about Stencil-managed files:

• What are they
• Where are they defined
• Prevent them from modifying them except in the appropriate locations.

[ekalinichev]

SHould we add markdown comments describing what we expect to be put in each section?