feat(agent-client): Codex process owner + per-surface views + pool

.changeset/codex-process-owner-pool.md

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+---
+"@pwrdrvr/agent-client": minor
+---
+
+Codex process owner + per-surface backend views + pool
+
+Adds `CodexProcessOwner` — one owned `codex app-server` process/connection that
+hands out lightweight per-surface `CodexBackendView`s (each an `AgentBackend`),
+so many chat surfaces share ONE Codex process. Every inbound notification and
+every tool-call/approval server-request is demultiplexed by `threadId` to the
+view that owns that thread, so sibling surfaces never clobber each other's
+handlers (plain `CodexThreadClient` exposes a single global handler slot — that
+clobber is exactly what this fixes). The owner also exposes `listModels()` and a
+structured `runOneShot()` (outputSchema, local images, per-turn rollback, token
+usage, abort) over the SAME connection, so model-picker refreshes and capture
+enrichment no longer each spawn their own App Server.
+
+`CodexProcessOwnerPool` mirrors `AcpAgentClientPool` (`acquire`/`warm`/`has`/
+`release`/`closeAll`), keyed by the host on connection identity (command,
+`CODEX_HOME`/env), giving one process per key with deduped concurrent warm-ups
+and app-level shutdown.
+
+`CodexThreadClient` and `CodexOneShotClient` are now thin shims over the owner
+(a single default view, and `runOneShot`/`listModels` respectively). Their public
+APIs and behavior are unchanged — additive release.