fix(agent): 子 agent 继承技能索引 —— 修 agent 把终端当命令执行器用

## 缺陷（生产实测）

`SetSkillIndexProvider` 只在 cmd/homed/bootstrap.go 里对**根 agent** 调了一次。
rtchild / 驻留子各自 `New(AgentConfig{...})` 构造载体，都没传这个字段 ⇒
子的 system prompt 里**完全没有 skill 索引**。

症状（日志实录）：A2A 会话由「临时分诊助手」（驻留子）接手，它手上没有
`cmd_run`（cmd 插件 `auto_start=false`），而 `cmd-tools` skill 正文里写着
「先 plugin_start cmd」。它看不到这条 skill，于是把 `terminal_create` 当成
命令执行器：每条 `cd … && echo … && grep …` 开一个新终端，终端随命令结束退出，
`terminal_read` 立刻撞 `read /dev/ptmx: input/output error`，然后为下一条命令
再开一个 —— 终端列表被填满，每一条都死在半路。

与本仓 Security 那次同型：父有的能力，子没继承。

## 修法

rtchild 与驻留子的 AgentConfig 各补一行 `SkillIndexProvider: a.skillIndex`，
与 `SystemPrompt` / `StageHost` / `Security` 同一组「必须继承父」的字段。

## 顺带两件事

### 1. 内置 skill 的更新永远到不了已部署机器

原落盘规则是「目标目录已存在就跳过」，本意是保住用户改动。但它把两件事混成一件：

    用户改过的版本     ⇒ 应该保留
    内核自己写过的旧版 ⇒ 应该随内核更新

两者都表现为「目录已存在」，于是 b850fea 把 `memory_recall` 改成
`stablemem_recall` 后，生产机的 kernel-memory 一直停在旧名，模型照着
旧名调工具必然失败，而日志里一个字都没有。

改为用 `.builtin-sha256` 摘要区分：
  当前内容 == stamp  ⇒ 没人动过 ⇒ 可安全覆盖成新版
  当前内容 != stamp  ⇒ 用户改过 ⇒ 保留
  没有 stamp（老目录）⇒ 保守视为用户版本，不覆盖

### 2. 新增 kernel-terminal skill

工具描述只说「适用于交互式程序」，没说何时**不该**用；`command` 参数又鼓励
传整条命令。补一条内置 skill 讲清 cmd_run 与 terminal_create 的边界、
以及终端的 input/key/notify 用法。描述控制在 80 字符内（进索引不截断）。

## 判据（均经变异验证）

  TestRTChild_InheritsSkillIndex        删掉接线 ⇒ 红（实测确认）
  TestResident_InheritsSkillIndex       同上（生产实际中招的那一类）
  TestRootAgent_SkillIndexStillInjected 确认没改坏根 agent 路径
  TestChildWithoutSkillIndexIsNotBroken 父无索引时子不 panic
  TestMaterializeBuiltinSkills_*        5 条：覆盖语义四条成组断言
                                        （改回旧行为 ⇒ OverwritesUnmodified 红，实测确认）

判据测的是**生成的 prompt**，不是 `skillIndex != nil`：
后者接上了但注入那段被删照样绿。

## 验证

  go build ./...                       PASS
  go vet ./...                         PASS
  gofmt -l                             干净
  go test ./... -count=1               62 包全过，0 FAIL
  干净检出（git worktree）单独验证      PASS

internal/plugins/skillmgr/builtin/kernel-memory/SKILL.md

@@ -16,7 +16,7 @@ Four stores, one rule each:
 ## Retrieving
 
 | Need | Tool |
-|---|---|
+| --- | --- |
 | A past episode in full | `coldcontext_query` |
 | A fact or relation | `stablemem_recall` |
 | Written reference material | `knowledge_search` |
@@ -33,5 +33,5 @@ again later, it lives on in the timeline until pruning moves it back.
 ## Writing
 
 - Facts worth keeping: `stablemem_commit` (triples).
-- Written knowledge: `knowledge_write`.
+- Written knowledge: `knowledge_create`.
 - Do not commit transient chatter, tool output, or your own status lines.

internal/plugins/skillmgr/builtin/kernel-terminal/SKILL.md

@@ -0,0 +1,65 @@
+---
+name: kernel-terminal
+description: Terminal waits for input; it is NOT cmd_run. Read before using vim/ssh/top.
+version: 1.0.0
+---
+
+# Terminals vs one-shot commands
+
+Two different tools. Picking the wrong one looks like it works, then fails in a way that
+points somewhere else.
+
+| Tool | Use for | Lifetime |
+| --- | --- | --- |
+| `cmd_run` | **One** command, then done: queries, scripts, `cd && … && grep …`, pipes, redirects, heredocs | Process exits when the command exits |
+| `terminal_create` | A program that **keeps running and keeps asking for input**: `vim`, `ssh`, `top`, `less`, an interactive REPL | Stays open until you `terminal_close` or its `timeout` expires |
+
+## The failure this prevents
+
+Running `terminal_create` with a compound one-shot command
+(`cd /x && echo … && ls … && grep …`) creates a terminal that exits the moment the command
+finishes. Reading it then fails with `read /dev/ptmx: input/output error`. The next instinct is
+to create another terminal for the next command — and now every "command" costs a terminal,
+fills the terminal list, and each one dies mid-read.
+
+**If the command runs to completion on its own, it belongs in `cmd_run`.** A terminal is only
+justified when the thing you are talking to is *still running and waiting for you*.
+
+## Driving a terminal
+
+`terminal_create` returns an ID. From then on the terminal is **silent by default** — it does
+not push output at you.
+
+1. `terminal_read(id)` — read what is on screen now. Do this after acting, not on a timer.
+2. `terminal_write(id, input=..., key=...)` — send text and/or a key (`Enter`, `Escape`, `C-c`)
+   in **one call**. `input` without `key` does not submit; the program just sits there.
+3. `terminal_close(id)` — release it. Terminals default to a 5-minute `timeout`; leaving them
+   open wastes a slot.
+
+`notify` is opt-in. Without it you get nothing. Add it only when you genuinely want a wake-up
+(`exit` for "tell me when it finished", `buffer=8192` for "tell me when there is a lot of
+unread output"). Watching a fast-scrolling program with `interval` produces noise, not signal.
+
+## Sending keystrokes
+
+`key` is for **control**, `input` is for **text**:
+
+| Want | Send |
+| --- | --- |
+| Submit a command line | `input="ls -la"`, `key="Enter"` |
+| Interrupt a running program | `key="C-c"` |
+| Leave a pager or editor without saving | `key="Escape"`, then `key="q"` for pagers |
+| Quit `vim` without saving | `key="Escape"`, then `input=":q!"`, `key="Enter"` |
+
+## Reading output that scrolled past
+
+`terminal_read` returns what is currently visible plus recent scrollback. If the output you need
+scrolled out of range, do not blindly re-read in a loop — either grow `rows` at create time, or
+for one-shot inspection prefer `cmd_run`, whose result is returned whole.
+
+## Checklist before reaching for a terminal
+
+- Does the command finish by itself? → `cmd_run`.
+- Does it need me to type at it more than once? → `terminal_create`.
+- Am I about to create a second terminal for "the next command"? → stop; the first one was
+  the wrong tool.