feat: add persistent PTY sessions

packages/pty/pty/README.md

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+# @deepseek-ai/dsh-pty
+
+Owner-scoped persistent PTY seam. `PtyService` registers as `ctx.pty`, mints opaque session ids, routes creation through named backends, fences every operation to the exact live `Agent`, and awaits backend quiescence when that agent or the service disposes.
+
+## Contract
+
+- Backends register one stable `type` and return an unpublished `PtyBackendSession`; failed or cancelled setup must clean partial resources.
+- A successful spawn publishes one `PtySessionId`. The optional `name` is owner-local display metadata, never authority.
+- One session accepts at most one live send operation. Reads and signals may observe it; another send fails until the operation settles.
+- `PtySendResult.waitReason` and `sessionStatus` are independent. `session_exit` describes the top-level PTY process, not an arbitrary foreground command.
+- `kill()` and disposal resolve only after the backend's captured process tree is quiescent. A cleanup failure rejects instead of claiming success.
+
+The seam contains no `node-pty`, sandbox, tool-schema, prompt, task, or terminal-rendering policy. Implementations own terminal mechanics; consumers own model presentation and optional background-task registration.
+
+## Model Experience
+
+### Indirect consumer
+
+#### What the model sees
+
+Nothing directly. This package registers no prompt or tool; `@deepseek-ai/dsh-tool-pty` owns visible schemas and result text.
+
+#### Token effect
+
+None directly. Live session state stays process-local until a consumer returns a bounded result.
+
+#### KV Cache effect
+
+No direct invalidation; the named consumer owns request-prefix changes.
+
+## Known Limitations and Deferred Work
+
+- Sessions are process-local and are not restored after a harness restart.
+- Cross-agent sharing is intentionally absent; a future shared-session design needs a separate authority contract.