refactor(agent): convert printer to plugin pattern

[zastrowm]

Summary

Refactors AgentPrinter from a standalone interface-based implementation to a first-class Plugin. This aligns with the existing plugin architecture and enables printer functionality to be cleanly integrated into the agent's hook system.

Key Changes

printer.ts

• Removed Printer interface - No longer needed with plugin-based approach
• AgentPrinter now implements Plugin interface with:
  • name property: 'strands:printer'
  • initAgent() method that registers callbacks via agent.addHook()
• Hooks registered:
  • ModelStreamUpdateEvent → handles streaming text/reasoning output
  • ToolResultEvent → handles tool completion status

agent.ts

• Removed _printer field - No longer managed separately
• Printer registered as plugin in PluginRegistry when enabled
• Removed manual processEvent() calls - Now handled automatically by hook system

Test Updates

• Updated printer configuration tests to use plugin pattern
• Replaced (agent as any)._printer access with proper plugin injection
• Use vi.spyOn(process.stdout, 'write') for stdout capture tests

Why This Approach

1. Consistency: Aligns with existing plugin architecture (e.g., ConversationManager, SessionManager)
2. Cleaner separation: Printer logic decoupled from Agent internals
3. Extensibility: Users can create custom printers by implementing Plugin
4. Testability: Plugin injection allows for clean test mocking

Resolves #44

[zastrowm]

/strands

[github-actions]

Issue: PR template has a "Documentation PR" section that should be included.

Suggestion: Add a "Documentation PR" section with a brief justification. Since this is an internal refactor with no public API changes, a simple note like "Not required - internal refactor with no public API changes" would suffice.

[github-actions]

Assessment: Comment

Clean refactor that correctly converts the printer to the Plugin pattern, improving architectural consistency.

Review Summary

• Implementation: AgentPrinter properly implements Plugin interface with name getter and initAgent() method. Good cleanup removing the unused Printer interface.
• Testing: Author has already identified the need to use vitest mocking instead of manually modifying process.stdout.write (lines 486, 508) - agree this should be addressed for consistency with repo patterns.
• Documentation: PR description is missing the "Documentation PR" section from the template. Since this is an internal refactor with no public API changes, a brief justification note would be sufficient.

The Plugin implementation follows established patterns well. Once the testing improvements noted by the author are addressed, this looks good to merge.

[zastrowm]

/strands merge in main and fix the conflicts

[zastrowm]

/strands - there's a bunch of changes that are not related to this PR; can you identify what happened and reverse the changes