Why

Multi-agent v2 treats agents as durable logical agents, not just live entries in ThreadManager. After the reload-on-delivery change, a v2 agent can be addressed even if its thread is not currently loaded.

This PR adds the next layer: loaded v2 subagents can be paged out of ThreadManager when the session has too many resident agents. That keeps residency separate from logical identity and prepares the stack for making v2 concurrency count active execution instead of existing agents.

What Changed

• Add an AgentControl-scoped LRU for resident v2 subagents.
• Reserve residency before spawning or reloading a v2 subagent.
• If resident capacity is full, unload the least-recently-used idle v2 subagent from ThreadManager.
• Keep ThreadManager as a primitive loaded-thread store; it does not own the LRU policy.
• Keep unloaded agents registered and durable so they can be reloaded by the delivery path.
• Preserve the existing v2 cap semantics by using the derived non-root v2 cap for residency.

Eviction is intentionally conservative. A thread is unloadable only when it is a v2 subagent, has completed or errored, has no active turn, and has no pending mailbox work. Before removal, the rollout is materialized and flushed.

Assumptions And Non-Goals

• PR feat: reload v2 agents on delivery #26623 provides the reload-on-delivery path for unloaded v2 agents.
• ThreadManager membership means loaded/resident, not logical agent existence.
• AgentRegistry remains the logical identity/metadata source for v2 agents that may be unloaded.
• list_agents remains a recent/resident view for now.
• This does not change active execution concurrency; that is the next PR.
• This does not change close_agent semantics.
• This does not change or remove resume_agent.
• This does not add a new residency config knob.

Stack

1. V2 durable lookup and reload on delivery (feat: reload v2 agents on delivery #26623) - reload unloaded v2 agents before delivering follow-up/input.
2. V2 residency LRU (this PR) - unload idle resident v2 agents from ThreadManager when resident capacity is full.
3. V2 active-execution concurrency - count running non-root v2 turns instead of logical agents.
4. V2 close/interrupt semantics - make v2 close interrupt the current turn without deleting durable identity.
5. V2 resume cleanup - remove the manual resume surface for v2 while keeping internal reload support.

Validation

• Added focused coverage for the residency LRU eviction path.
• Local clippy/check/tests were not run; CI will cover them.