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.

examples/package.json

@@ -10,6 +10,7 @@
     "@deepseek-ai/dsh-acp-demo": "workspace:*",
     "@deepseek-ai/dsh-agent-spine-demo": "workspace:*",
     "@deepseek-ai/dsh-app-boot": "workspace:*",
+    "@deepseek-ai/dsh-bash": "workspace:*",
     "@deepseek-ai/dsh-bash-local": "workspace:*",
     "@deepseek-ai/dsh-bash-sandbox": "workspace:*",
     "@deepseek-ai/dsh-cli-demo": "workspace:*",
@@ -54,6 +55,7 @@
     "@deepseek-ai/dsh-tasks-local": "workspace:*",
     "@deepseek-ai/dsh-time-context": "workspace:*",
     "@deepseek-ai/dsh-timeout-policy": "workspace:*",
+    "@deepseek-ai/dsh-tmux-context": "workspace:*",
     "@deepseek-ai/dsh-token-meter": "workspace:*",
     "@deepseek-ai/dsh-tool-ask-user": "workspace:*",
     "@deepseek-ai/dsh-tool-cordis": "workspace:*",