## 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 #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 (#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.
## Summary
This is the first small step toward making multi-agent v2 agents durable
logical agents whose `ThreadManager` residency is only an implementation
detail.
This PR adds a narrow v2 reload-on-delivery hook:
- If a known v2 agent target is already loaded, delivery is unchanged.
- If the target is still registered but missing from `ThreadManager`,
delivery reloads that exact v2 thread from durable rollout history
before submitting the message.
- If the target is unknown, closed, missing from storage, or not a v2
thread, delivery still fails as not found.
The reload is wired only into existing-agent delivery paths: v2
`send_message` / `followup_task`, and legacy `send_input` when its
target is a known v2 agent.
## Stack
1. **Reload on delivery**: load known unloaded v2 agents before
`followup_task`, `send_message`, or `send_input` delivery. This PR.
2. **Residency LRU**: unload idle resident v2 agents from
`ThreadManager` without making them closed or unreachable.
3. **Execution concurrency**: count active non-root turns, not logical
agents or resident idle threads.
4. **Close semantics**: make v2 close interrupt-only and leave durable
agent identity intact.
5. **Resume cleanup**: remove user-facing v2 resume semantics;
addressing an unloaded durable agent reloads it implicitly.
## Validation
- Ran `just fmt`.
- Left broader tests and clippy to CI.
## Summary
Mechanically splits `AgentControl` into focused modules so later agent
runtime changes are easier to review. The shared lookup, messaging, and
completion logic remains in `control.rs`, while spawn-specific code and
V1 legacy close/resume behavior move into dedicated files.
## Changes
- Extract spawn-agent code into `agent/control/spawn.rs`.
- Extract V1-only legacy close/resume behavior into
`agent/control/legacy.rs`.
- Keep shared control-plane behavior in `agent/control.rs`.
- Preserve existing behavior; this PR is intended to be mechanical.
## Stack
1. This PR - Mechanical `AgentControl` split: extracts spawn and V1
legacy code without behavior changes.
2. #26614 - Execution slot accounting: separates logical agents from
active execution slots.
3. #26611 - Residency and reload runtime: adds resident-agent LRU,
eviction/reload, durable lookup, and V2 delivery through reload.
4. #26612 - V2 tool semantics: narrows `close_agent` to interrupt-only
and updates V2 tool coverage.
jif