feat(memory): Wire inject_memories through runtime AgentDefaults

[hamza-jeddad]

Background

Sub-issue of #3011. Connect the config fields (from the config sub-issue) to the builtin (from the scaffolding sub-issue) so the runtime actually activates inject_memories for agents that opt in.

Scope

Wire the new AgentConfig fields through the existing builtins/runtime plumbing:

AgentConfig (YAML)
   │
   ▼
ApplyAgentDefaults  → builtins.AgentDefaults
   │
   ▼
buildHooksExecutors → registers inject_memories handler when enabled

Implementation Checklist

• pkg/agent/agent.go — add accessors:
  • InjectMemories() bool
  • MaxInjectMemories() int
  • InjectMemoriesStrategy() string
• pkg/runtime/hooks.go — ApplyAgentDefaults (or equivalent) maps AgentConfig fields to builtins.AgentDefaults
• pkg/runtime/hooks.go — buildHooksExecutors registers the inject_memories turn_start handler when AgentDefaults.InjectMemories == true and a memory toolset is bound to the agent
• Populate hooks.Input.LastUserMessage in executeTurnStartHooks (also tracked in feat(memory): Design and implement inject_memories retrieval strategy #3013) — required for the retrieval pipeline to see the user's prompt
• Unit tests for wiring: config in → handler registered (or not) → executor list contains inject_memories (or does not)

Acceptance Criteria

• An agent with inject_memories: true and a memory toolset gets the handler registered in its turn_start executors
• An agent with inject_memories: true but no memory toolset logs a warning and does not register the handler (graceful degradation)
• An agent with inject_memories: false (or unset) has zero behavioural change
• Input.LastUserMessage is populated for all turn_start hooks (mirrors executeStopHooks pattern)
• No regressions in existing turn_start builtin ordering