feat(context): add tmux-context plugin injecting the agent's tmux location

Add @deepseek-ai/dsh-tmux-context: an opt-in per-turn context plugin that
reads which tmux session/window/pane this agent process runs in (plus the
window layout tree) via the ctx.bash seam, and injects it as one durable,
source-attributed user/message when the location changes.

- Pull on the first step of each turn; no tmux hook or background process.
- Detect a real pane by tty, not $TMUX_PANE alone: a terminal launched from
  a tmux shell inherits $TMUX/$TMUX_PANE from that ancestor, so the command
  also matches the pane's #{pane_tty} against this process's controlling
  terminal and emits fields only on a match.
- No-op outside a real pane, without a bash executor, or on a malformed
  reading.
- Own location and layout only: no pane sizes, no sibling-pane scraping.
- Unit tests at 100% per-file coverage, plus a keyless Loader e2e with a
  mock bash provider so it replays without tmux.
- Agent Note: 2026-07-27-tmux-location-context.

packages/context/README.zh.md

@@ -2,12 +2,13 @@
 
 [English](README.md) | 中文
 
-这些产品插件无需定义工具，即可增加模型可见的请求上下文。`workspace-context` 包含在默认的 `dsh-agent-spine-demo` 组合包中，且可通过组合包配置将其禁用；`time-context` 需要选择启用，标准 TUI 组合包则会显式组合 `session-reference`。
+这些产品插件无需定义工具，即可增加模型可见的请求上下文。`workspace-context` 包含在默认的 `dsh-agent-spine-demo` 组合包中，且可通过组合包配置将其禁用；`time-context` 与 `tmux-context` 需要选择启用，标准 TUI 组合包则会显式组合 `session-reference`。
 
 | 包 | 职责 | ctx key |
 |---|---|---|
 | `session-reference/` | 其他会话当前表层的有界快照 | `ctx.sessionReferences` |
 | `time-context/` | 持久的逐步骤当前时间与耗时上下文 | （无） |
+| `tmux-context/` | 持久的逐轮上下文，记录本 agent 所在的 tmux pane/window 位置 | （监听 `agent/step`，读取 `ctx.bash`） |
 | `workspace-context/` | `AGENTS.md`／`CLAUDE.md` 工作区上下文 loader | （监听 `agent/step` + `tools/post-execute`） |
 
 [`workspace-context` 决策记录](../../.agents/notes/implemented/feature/2026-06-24-workspace-context.md)解释了它的逐 agent／会话隔离与生命周期拆分。