Architecture documentation

[andres-javier-lopez]

Description

Add documentation for external AI Agents or developers to easily implement the library

Type of change

• Bugfix
• New feature
• Documentation update
• Refactor
• Chore

Checklist

Please run through the checklist and mark the items off before requesting review.

• Tests added/updated for changes in behavior
• Linter/formatter run (make format / make lint)
• Documentation updated (if applicable)
• CHANGELOG.md updated with a short entry describing the change (Required)

[Copilot]

Pull request overview

Adds architecture and external-consumer build documentation to help downstream users understand assistant_core’s builder/context model and implement agents more consistently.

Changes:

• Add new internal architecture guide (ARCHITECTURE.md) describing core layers (state/nodes/factories/context/builders) and build pipeline.
• Add new external “from-scratch” agent build reference (AGENT_BUILD_REFERENCE.md) with recommended patterns (ContextFactory/BuilderContext, tool builders, entrypoint chaining).
• Update README and CHANGELOG to point to the new documentation and reflect current module layout/extension points.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
README.md	Adds links to new docs and updates project layout/extension-point guidance.
CHANGELOG.md	Records the documentation additions under NEXT.
ARCHITECTURE.md	New internal architecture/reference document for builders, nodes, and state flow.
AGENT_BUILD_REFERENCE.md	New prescriptive guide for external library consumers to build agents with assistant_core.
.github/copilot-instructions.md	Updates contributor/copilot guidance around verification after formatting/linting.

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