feat: add persistent PTY sessions

packages/pty/tool-pty/README.md

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+# @deepseek-ai/dsh-tool-pty
+
+Six model-facing tools over `ctx.pty`: `pty_spawn`, `pty_send`, `pty_read`, `pty_signal`, `pty_kill`, and `pty_list`. Every operation requires the exact initiating `Agent`, so a model cannot address another agent's terminal even if it learns the id.
+
+`pty_send(run_in_background: true)` reuses `ctx.tasks`; task preflight occurs before any terminal write, completion is collected with `task_output`, and `task_kill` requests `Ctrl-C`. Foreground sends use terminal ACP cards; lifecycle, history, signal, and list calls use generic cards.
+
+## Model Experience
+
+### System prompt
+
+#### What the model sees
+
+The plugin contributes this fixed guidance section:
+
+##### PTY guidance
+
+```markdown
+Use PTY only when work needs persistent terminal state or interactive stdin; prefer bash/read/write/edit for bounded one-shot operations. Track every PTY session id and kill sessions that no longer matter. An inferred_idle or timeout result does not prove the foreground command exited.
+```
+
+#### Token effect
+
+Small fixed input cost on every request while the plugin is active.
+
+#### KV Cache effect
+
+Prefix-stable while the registration scope and guidance text are unchanged.
+
+### Tool schemas
+
+#### What the model sees
+
+The six generated schemas are listed in the [`dsh-tool-pty` catalog section](../../../docs/tool-catalog.md#deepseek-aidsh-tool-pty). Their fixed schema tokens are present whenever this plugin is active; agent-scoped tool filtering may hide them.
+
+#### Token effect
+
+Fixed schema cost on requests where the tools are visible.
+
+#### KV Cache effect
+
+Prefix-stable while tool visibility and definitions are unchanged.
+
+### Tool results and task context
+
+#### What the model sees
+
+Spawn returns the id and bounded MOTD. Send/read return bounded terminal text plus readiness/history markers. Background mode returns a generic task id. Results remain in session history until compaction; incremental task reads do not repeat consumed output.
+
+#### Token effect
+
+Data-dependent and bounded by the backend; each returned result remains in history until compaction.
+
+#### KV Cache effect
+
+Append-only; new results follow the reusable request prefix.
+
+## Known Limitations and Deferred Work
+
+- No named key sequence, TUI, BEL, resize, auto-start, or cross-agent sharing schema is exposed.
+- Background mode requires both `@deepseek-ai/dsh-tasks` and its model-facing control surface.