refactor(runner): ensure message author is set and improve history handling

[tonybase]

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[Copilot]

Pull request overview

This PR refactors the session message management by moving message persistence responsibilities from individual agents to the Runner, and simplifies the action/interruption handling system.

• Consolidates message persistence logic in the Runner rather than distributed across agents
• Removes the ResumeHistory configuration option and related functionality
• Moves the ActionHandoffToAgent constant to the internal/handoff package and removes the now-unused ActionInterrupted constant and Interrupted() helper function
• Replaces middleware-specific error (ErrConfirmDenied) with a common ErrInterrupted error

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
runner.go	Removes ResumeHistory field, adds nil check for messages in buildInvocation, updates Run to persist completed messages, refactors RunStream to use invocation-based history filtering, and renames historySets to historyByInvocation with corrected filtering logic
agent.go	Removes calls to appendMessageToSession from the handle method, replacing them with simple Author field assignment, and removes early termination checks for interrupted actions
action.go	Deletes the entire file containing ActionInterrupted, ActionHandoffToAgent constants and Interrupted() helper function
internal/handoff/handoff.go	Adds ActionHandoffToAgent constant (moved from action.go) and updates usage to reference the local constant
flow/routing.go	Updates import to reference handoff.ActionHandoffToAgent instead of blades.ActionHandoffToAgent
middleware/confirm.go	Removes ErrConfirmDenied error definition and updates to use blades.ErrInterrupted instead
errors.go	Adds new ErrInterrupted error for indicating interrupted execution

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