feat: add agents files

[mr-cal]

Adds an:

• Agents template for craft apps
• Agents template for craft libraries
• Agents file for starbase itself

Sources used:

• Evaluating AGENTS.md: Are Repository-Level Context Files Helpful for Coding Agents? (source)
• On the Impact of AGENTS.md Files on the Efficiency of AI Coding Agents (source)
• Prompting best practices from the Claude API docs (source)
• https://agents.md/
  • This is the official standard for agents files. It covers the file name, loosely covers file discovery, and doesn't really cover file contents.

(CRAFT-5188)

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• I've followed the contribution guidelines.
• I've signed the CLA.
• I've successfully run make lint && make test.
• I've added or updated any relevant documentation.
• In documents I changed, I added a meta description if one was missing.
• I've updated the relevant release notes.

[Copilot]

Pull request overview

Adds repository-level agent context (AGENTS.md) plus two reusable agent-file templates for downstream Craft apps and libraries, and updates the project template checklist to reference them.

Changes:

• Added AGENTS.md describing Starbase context, development commands, and contribution practices for coding agents.
• Added AGENTS.app.md and AGENTS.lib.md templates for downstream Craft apps/libraries to customize and rename to AGENTS.md.
• Updated .github/README.md “Create a new project” checklist to include the new agents template step and cleanup instruction.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
AGENTS.md	Adds Starbase-specific agent context and common dev/test/lint commands.
AGENTS.app.md	Provides an app-focused AGENTS template for Craft Application-based CLIs.
AGENTS.lib.md	Provides a library-focused AGENTS template for craft libraries.
.github/README.md	Updates new-project checklist to incorporate the agents templates and cleanup steps.

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[mr-cal]

RTD failure unrelated

[lengau]

Out of scope, but a "merge starbase" skill would be really cool.

[mr-cal]

Agreed! We can mention the canonical/copilot-collections#43 skill when it lands.