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.

tsconfig.host.json

@@ -80,6 +80,7 @@
     { "path": "./packages/goal/goal-session" },
     { "path": "./packages/goal/command-goal" },
     { "path": "./packages/context/time-context" },
+    { "path": "./packages/context/tmux-context" },
     { "path": "./packages/context/session-reference" },
     { "path": "./packages/ui/user-interaction" },
     { "path": "./packages/ui/user-approval" },