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.

tsconfig.host.json

@@ -83,6 +83,7 @@
     { "path": "./packages/goal/tool-goal" },
     { "path": "./packages/goal/goal-session" },
     { "path": "./packages/goal/command-goal" },
+    { "path": "./packages/feedback/command-feedback" },
     { "path": "./packages/context/time-context" },
     { "path": "./packages/context/session-reference" },
     { "path": "./packages/ui/user-interaction" },