perf(agents): pool shared harness runtimes

[Waishnav]

Provider adapters currently create expensive runtime resources for individual turns. This adds a small server-owned runtime registry and pool, then proves the sharing model with OpenCode: compatible agents reuse one OpenCode server while active work prevents eviction and idle runtimes are reclaimed.\n\nThe pool only owns live resources; logical agent identity and durable continuation state remain outside it. Provider-specific sharing policy stays behind each driver runtime key. Stacked on #190.

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[greptile-apps]

Greptile Summary

The PR introduces a shared, idle-reaped runtime pool for OpenCode subagents and wires its lifecycle into the server.

• Reuses one OpenCode harness across compatible runs
• Reclaims idle runtimes and closes them during server shutdown
• Routes other providers through their existing one-shot adapters
• Adds coverage for runtime reuse, eviction, and recreation

Confidence Score: 4/5

The OpenCode pooling path should be fixed before merging because an unexpectedly dead harness remains cached and can make every later turn fail.

The new pool supports replacement only when a runtime reports itself unusable, while the OpenCode implementation continues reporting usable after server or transport failures unless the pool explicitly closes it.

Files Needing Attention: src/local-agent-adapters.ts, src/local-agent-runtime-pool.ts

Important Files Changed

Filename	Overview
src/local-agent-runtime-pool.ts	Adds shared runtime acquisition, idle reaping, and shutdown handling; failed runs rely on the runtime implementation to report itself unusable.
src/local-agent-adapters.ts	Extracts OpenCode into a reusable harness runtime, but its usability state does not reflect an unexpectedly failed server.
src/local-agent-runtime-registry.ts	Routes OpenCode through the pool while preserving one-shot behavior for other providers.
src/server.ts	Creates the runtime registry and shuts it down after queued local-agent work drains.
src/local-agent-runtime-pool.test.ts	Covers reuse and idle eviction but not recovery after the underlying harness fails.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  Request[Subagent request] --> Manager[LocalAgentManager]
  Manager --> Registry[LocalAgentRuntimeRegistry]
  Registry -->|OpenCode| Pool[HarnessRuntimePool]
  Registry -->|Other provider| Adapter[One-shot adapter]
  Pool -->|Acquire or create| Runtime[OpenCode harness runtime]
  Runtime --> Result[Provider result]
  Result --> Manager
  Reaper[Idle reaper] -->|Close idle runtime| Runtime
  Shutdown[Server shutdown] --> Manager
  Shutdown -->|After manager drains| Pool

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