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.

knip.json

@@ -36,6 +36,9 @@
         "headless-agent/tests/fixtures/time-context-mock-llm.ts",
         "headless-agent/tests/fixtures/telemetry-otel-driver.ts",
         "headless-agent/tests/fixtures/telemetry-redact-rule.ts",
+        "headless-agent/tests/fixtures/tmux-context-driver.ts",
+        "headless-agent/tests/fixtures/tmux-context-mock-llm.ts",
+        "headless-agent/tests/fixtures/tmux-context-mock-bash.ts",
         "acp-agent/tests/snapshots/lsp-definition/workspace/subject.ts",
         "tui-agent/tests/fixtures/tui-scripted-llm.ts",
         "acp-agent/tests/fixtures/subagent/subagent-acp/mock-delegating-llm.ts",
@@ -163,6 +166,16 @@
         "tests/**/*.ts"
       ]
     },
+    "packages/context/tmux-context": {
+      "entry": [
+        "tests/**/*.spec.ts",
+        "tests/**/*.e2e.ts"
+      ],
+      "project": [
+        "src/**/*.ts",
+        "tests/**/*.ts"
+      ]
+    },
     "packages/lsp/lsp-local": {
       "entry": [
         "tests/**/*.spec.ts",