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>

CHANGELOG.md

@@ -38,6 +38,14 @@ instruction file on the way down to where it was started. Harness spec 3.0.0.
 - **A chat started on a model runs on that model.** When the web UI opens a session on this device
   with a model the device cannot run, it is refused with the reason instead of being answered on
   the device's own model.
+- **File tools speak the server's words.** A missing file is `No such file or directory: <path>`
+  from `read`, `edit`, `multiedit`, `show`, `list`, `glob` and `grep` alike; a file `write` or
+  `edit` may not change yet says it has not been read, or changed since it was last read, with its
+  full path. `glob` and `grep` give full paths. `list` sorts names exactly (capitals first) and says
+  when it stopped at 400 lines.
+- **A new skill's name** is lowercase letters, digits and `-`, up to 64 characters, as the Agent
+  Skills format has it; `_` and `.` are no longer accepted when `skill_manage` makes one. Skills
+  that already have such a name still load.
 
 ### Added