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.base.json

@@ -72,6 +72,7 @@
         "./packages/compact/*/src/invariant.ts",
         "./packages/context/*/src/invariant.ts",
         "./packages/goal/*/src/invariant.ts",
+        "./packages/feedback/*/src/invariant.ts",
         "./packages/guard/*/src/invariant.ts",
         "./packages/plan/*/src/invariant.ts",
         "./packages/subagent/*/src/invariant.ts",
@@ -161,6 +162,7 @@
         "./packages/compact/*/src",
         "./packages/context/*/src",
         "./packages/goal/*/src",
+        "./packages/feedback/*/src",
         "./packages/guard/*/src",
         "./packages/plan/*/src",
         "./packages/subagent/*/src",