Harness spec: one path form, the file tools' texts, the list tree, skills

filesystem.md, spec 3.0.0-draft, and the CLI made to match:

- Paths: absolute in every answer except a result's bracket (context.md),
  which keeps the call's path. glob and grep now give full paths.
- A missing file is `No such file or directory: {path}` for every tool
  that needs it, with no hint after it: in a chat a hidden file must read
  as a missing one. The CLI's "does not exist. Use write to create it." and
  "Write it first." are gone; an edit of a directory is `Is a directory`,
  glob over a file `Not a directory`.
- write/edit refusals, exactly: "{path} exists and has not been read in
  this session. Read it first." and "{path} changed since you last read
  it. Read it again before changing it." (no "on disk"), absolute path.
- A listing sorts names in code-point order; `list` with a depth is a
  tree indented two spaces a level, marks as in a single listing, cut at
  400 lines with a line saying so.
- Skills: where they are found and in what order on each side (the
  session's own place first, the person's next, what is shared last), and
  the name rule ^[a-z0-9][a-z0-9-]{0,63}$ that skill_manage checks. The
  CLI no longer makes names with _ or .; such skills still load.

README's 3.0.0 notes say so; CHANGELOG [Unreleased].

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/tool/show.ts

@@ -59,11 +59,11 @@ export const showTool = defineTool({
     let size: number
     try {
       const st = statSync(abs)
-      if (!st.isFile()) throw new ToolError(`${showPath(abs, ctx)} is not a file.`)
+      if (!st.isFile()) throw new ToolError(st.isDirectory() ? `Is a directory: ${abs}` : `${showPath(abs, ctx)} is not a file.`)
       size = st.size
     } catch (e) {
       if (e instanceof ToolError) throw e
-      throw new ToolError(`${showPath(abs, ctx)} does not exist. Write it first.`)
+      throw new ToolError(`No such file or directory: ${abs}`)
     }
     if (size > SHOW_MAX_BYTES) throw new ToolError(`${showPath(abs, ctx)} is ${Math.round(size / 1024)} KB; at most 1 MB can be shown. Give the user the path instead.`)
     const text = await readText(abs)