feat(acp): Add E2E tests for agent subprocess lifecycle and fix cleanup (#83)

## Summary
🤖 Generated with [Nori](https://www.npmjs.com/package/nori-ai)

- Add 6 E2E tests in `agent_switching.rs` to verify ACP agent subprocess
handling:
  - Subprocess spawning with unique PIDs
  - Session switch via `/new` creates new subprocess
  - Old subprocess cleanup after switch (no zombies)
  - Cleanup timing (outside prompt turns)
  - Different agents use different subprocesses
  - Model picker switch creates new subprocess
- Add `mock-model-alt` to ACP registry for testing agent switching
- Fix subprocess cleanup: kill child process when `AcpConnection` is
dropped
- Fix Arc reference deadlock that prevented cleanup from happening

## Test Plan
- [x] All 6 new E2E tests pass (`cargo test -p tui-pty-e2e --test
agent_switching`)
- [x] Unit tests pass (`cargo test -p codex-acp`)
- [x] Clippy passes
- [x] Code formatted with `cargo fmt`

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Author: CSRessel

.claude/settings.json

@@ -4,6 +4,27 @@
     "additionalDirectories": [
       "/home/clifford/Documents/source/nori/cli/.claude/profiles",
       "/home/clifford/Documents/source/nori/cli/.claude/skills"
+    ],
+    "allow": [
+      "Bash(cat:*)",
+      "Bash(cd:*)",
+      "Bash(git status:*)",
+      "Bash(mkdir:*)",
+      "Bash(tree:*)",
+      "Bash(cargo build:*)",
+      "Bash(cargo b:*)",
+      "Bash(cargo check:*)",
+      "Bash(cargo c:*)",
+      "Bash(cargo clean:*)",
+      "Bash(cargo clippy:*)",
+      "Bash(cargo doc:*)",
+      "Bash(cargo d:*)",
+      "Bash(cargo fmt:*)",
+      "Bash(cargo run:*)",
+      "Bash(cargo r:*)",
+      "Bash(cargo test:*)",
+      "Bash(cargo t:*)",
+      "Bash(cargo search:*)"
     ]
   }
-}
+}