Add bash execution: dsh-bash seam, dsh-bash-local impl, dsh-tool-bash tools

Three packages following the new capability-seam pattern (interface /
implementation / consumer, now documented in docs/architecture.md):

- dsh-bash: abstract BashExecutor service (ctx.bash) + vocabulary types.
- dsh-bash-local: local subprocesses — bash -c per call in a detached
  process group, SIGTERM→SIGKILL group kills, tail-keep truncation with
  full-stream spill files, model-friendly env, background task registry.
- dsh-tool-bash: the bash / bash_output / bash_kill tool schemas with
  runtime arg validation and background completion notices via
  agent.inject(). Non-zero exits are reported, not errored.

Design surveyed against the bash tools of Claude Code, OpenCode, Codex,
and pi (notes in the package READMEs). Permissions/sandbox stay TODO on
the tools/execute waterfall seam; stateful-shell alternatives recorded
in run.ts.

tsconfig.build.json

@@ -15,6 +15,9 @@
     { "path": "./packages/system-prompt" },
     { "path": "./packages/agent" },
     { "path": "./packages/tools" },
-    { "path": "./packages/agent-loop" }
+    { "path": "./packages/agent-loop" },
+    { "path": "./packages/bash" },
+    { "path": "./packages/bash-local" },
+    { "path": "./packages/tool-bash" }
   ]
 }