feat(feedback): add a /feedback command recorded through the command plane

Register a global `/feedback` command so a user can record a remark about the
session without spending a model turn. `/feedback <text>` acknowledges; empty
or whitespace-only input returns a usage error.

The plugin appends no session event of its own. `dsh-commands` already writes
a `command/run` / `command/done` pair for every dispatched command, carrying
the verbatim text and the settled outcome, and both records are log-only and
non-surface. The feedback is therefore durably in the session log and invisible
to the model without this package touching the log format.

Text is never parsed, so `/feedback /plan felt slow` records that literal
content. Nothing consumes the records; capture is deliberately inert.

New group `packages/feedback/` — no existing group owns feedback capture. Its
row raises the packages/README.md word ceiling by 10, which had no headroom;
one redundant sentence there was removed to offset most of the cost.

packages/examples/tui-demo/package.json

@@ -32,6 +32,7 @@
     "@deepseek-ai/dsh-agent-loop": "^0.0.1",
     "@deepseek-ai/dsh-commands": "^0.0.1",
     "@deepseek-ai/dsh-command-goal": "^0.0.1",
+    "@deepseek-ai/dsh-command-feedback": "^0.0.1",
     "@deepseek-ai/dsh-agent-spine-demo": "^0.0.1",
     "@deepseek-ai/dsh-invariants": "^0.0.1",
     "@deepseek-ai/dsh-llm": "^0.0.1",
@@ -55,6 +56,7 @@
     "@deepseek-ai/dsh-agent-loop": "workspace:^",
     "@deepseek-ai/dsh-commands": "workspace:^",
     "@deepseek-ai/dsh-command-goal": "workspace:^",
+    "@deepseek-ai/dsh-command-feedback": "workspace:^",
     "@deepseek-ai/dsh-agent-spine-demo": "workspace:^",
     "@deepseek-ai/dsh-invariants": "workspace:^",
     "@deepseek-ai/dsh-llm": "workspace:^",