agentHost: retain recently used sessions

[roblourens]

Summary

• replace the 30-second idle release timeout with a soft MRU limit of 10 resident root sessions
• count active and observed sessions toward the limit while allowing pinned sessions to overflow it temporarily
• release archived and least-recently-used eligible sessions non-destructively, preserving lossless restore
• centralize logical subscriber liveness in an Agent Host subscription service
• isolate MRU and provider-release coordination in AgentSessionResidency

Validation

• npm run typecheck-client
• npm run valid-layers-check
• AgentSessionResidency unit tests: 8 passing
• AgentService unit tests: 362 passing, 16 pending
• Agent Host service bootstrap tests: 8 passing
• targeted session lifecycle integration test: 1 passing
• pre-commit hygiene hook

(Written by Copilot)

[Copilot]

Pull request overview

Replaces timeout-based idle eviction with MRU-based Agent Host session residency while preserving durable restoration.

Changes:

• Adds centralized subscription tracking and MRU residency coordination.
• Releases archived or excess eligible sessions with retry handling.
• Updates lifecycle, provider, and integration coverage for the new policy.

Show a summary per file

File	Description
common/agentHostSubscriptionService.ts	Defines subscription tracking and session resolution.
common/agentService.ts	Adds residency limit and retry environment variables.
node/agentHostSubscriptionService.ts	Implements centralized subscriber tracking.
node/agentSessionResidency.ts	Implements MRU eviction and provider release coordination.
node/agentService.ts	Integrates residency with session lifecycle operations.
node/agentServiceComposition.ts	Passes service options into AgentService.
node/agentHostServices.ts	Registers the subscription service.
node/agentHostStateManager.ts	Updates residency-related documentation.
test/node/agentSessionResidency.test.ts	Tests MRU, pinning, retries, and races.
test/node/agentService.test.ts	Updates and expands lifecycle coverage.
test/node/agentServiceTestUtils.ts	Exposes residency test options.
test/node/agentHostStateManager.test.ts	Updates eviction terminology.
test/node/protocolServerHandler.test.ts	Updates residency terminology.
test/node/protocol/sessionLifecycle.integrationTest.ts	Validates zero-capacity release and restore.
test/node/providerIntegration/copilotMockLlm.integrationTest.ts	Validates provider release with retry timing.
test/node/e2e/harness/agentHostE2ETestHarness.ts	Configures zero-capacity E2E servers.

Review details

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• Files reviewed: 16/16 changed files
• Comments generated: 0
• Review effort level: Balanced