feat(memory): Scaffold inject_memories turn_start builtin

[hamza-jeddad]

Background

Sub-issue of #3011. Create the inject_memories builtin file and register it as a turn_start hook, following the precedent set by add_date, add_environment_info, and add_prompt_files in pkg/hooks/builtins/.

This issue is scoped to the builtin scaffolding only — the retrieval logic itself is defined in #3013, and runtime wiring is handled in a separate sub-issue.

Scope

• Create pkg/hooks/builtins/inject_memories.go with the turn_start builtin signature
• Register a constant (e.g. InjectMemories = "inject_memories") and the handler in pkg/hooks/builtins/builtins.go
• Add the corresponding fields to AgentDefaults:
  • InjectMemories bool
  • MaxInjectMemories int
  • InjectMemoriesStrategy string
• The builtin must be a no-op for now if either the master toggle is off OR no memory toolset is present
• The actual keyword extraction + SearchMemories call is implemented in feat(memory): Design and implement inject_memories retrieval strategy #3013 (this issue lays the foundation)

Implementation Checklist

• pkg/hooks/builtins/inject_memories.go — file + handler skeleton
• pkg/hooks/builtins/builtins.go — register constant, handler, AgentDefaults fields
• Stub returns empty AdditionalContext until feat(memory): Design and implement inject_memories retrieval strategy #3013 lands
• Unit tests: builtin returns no-op when disabled; builtin returns no-op when no memory toolset attached
• godoc comments on exported symbols

Acceptance Criteria

• Builtin compiles and is discoverable through the builtins registry
• Disabled by default (AgentDefaults.InjectMemories == false)
• Calling the builtin with InjectMemories: false returns no AdditionalContext
• Calling the builtin without a memory toolset wired returns no AdditionalContext
• No behavioural change to any other builtin or to the default turn_start execution order