refactor(plan): remove generic mode abstraction

docs/cookbook/extension-cookbook.md

@@ -113,7 +113,7 @@ Every product feature maps to a listener on a documented extension seam — the
 | Monotonic terminal turn policy | return `{ action: 'stop' }` from serial `agent/turn-stop`, after continuation and steering have already been folded |
 | Subprocess sandbox (landlock / sandbox-exec) | use a `ctx.sandbox` backend through `dsh-bash-sandbox`; use `tools/pre-execute` for capability-level denial |
 | Permission system / AskUserQuestion | return `ask` from `tools/pre-execute` and answer through `ctx.approval`; register a separate model-facing ask tool for ordinary user questions |
-| Plan mode | Shipped: [`@deepseek-ai/dsh-mode`](../../packages/mode/mode/README.md) — the logged `mode/set` state, the `mode:policy` guidance section, and the user-reviewed `exit_plan_mode` exit; enforcement stays on the independent sandbox/approval axes |
+| Plan mode | Shipped: [`@deepseek-ai/dsh-plan-mode`](../../packages/plan/plan-mode/README.md) — logged `plan/mode` state, the `plan:policy` guidance section, `/plan [message]`, and the user-reviewed `exit_plan_mode` exit; enforcement stays on the independent sandbox/approval axes |
 | Sub-agent delegation | the `ctx.subagents` provider registry (`dsh-subagent-spawn`/`-fork`/`-acp`) + `dsh-tool-subagent` exposing one configured provider to the model |
 | MCP | one plugin per server: discover tools → `ctx.tools.register()` |
 | Skills | section + tool registration; `inject()` skill content on invocation |