refactor(agent): convert printer to plugin pattern

[zastrowm]

Description

AgentPrinter was wired directly into the agent loop as a private _printer field, with the agent manually calling _printer?.processEvent(event) on every yielded event. AgentPrinter now implements Plugin and registers its own hook callbacks via initAgent(). The Printer interface has been removed as it was only used internally.

The printer: false option still suppresses it and default behavior is unchanged.

Related Issues

#270

Documentation PR

Internal refactor; no public API changes requiring doc updates.

Type of Change

Refactor

Testing

How have you tested the change?

• I ran npm run check

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[github-actions]

Code Review: ✅ Approve

Assessment: Approve

Clean internal refactor that converts AgentPrinter from a directly-wired implementation to the Plugin pattern, improving extensibility and consistency with other SDK components like SlidingWindowConversationManager.

Review Summary

• Plugin Pattern: Correctly implements Plugin interface with name getter and initAgent() hook registration
• Backward Compatibility: printer?: boolean config maintained, default behavior unchanged
• Internal Only: Printer interface removed (was internal-only), AgentPrinter not exported from index.ts
• Test Quality: Tests updated to verify behavior externally via plugin injection rather than relying on internal _printer field access
• Code Quality: All checks pass (83 tests, type-check, lint)

Good work on maintaining the same functionality while making the architecture more consistent. 👍