Always replace the installed skill instead of preserving local edits

The skill directory is a CLI-managed artifact, so install now replaces it
wholesale and remove deletes it. Drops the manifest file, the content
hashing, and --force.

Preserving edits was worse than losing them. An edited file was skipped
by every future install, so it stayed frozen forever against a CLI that
keeps changing, which defeats the reason for embedding the skill in the
binary at all. Replacing wholesale also drops files an older version
shipped, with no reconciliation logic needed.

This removes the three ownership-loss paths raised in review rather than
patching them, since all of them came from keeping that bookkeeping file
in sync with what was on disk.

SKILL.md now says it is managed and points anywhere else for custom
guidance.

crates-cli/yaak-cli/skills/use-yaak/SKILL.md

@@ -19,6 +19,12 @@ allowed-tools: Bash(yaak:*), Bash(which:*), Bash(command:*), Bash(npm:*), Bash(n
 
 # Use Yaak
 
+<!--
+  Managed by the Yaak CLI. `yaak agent install` replaces this file wholesale on
+  every run, so local edits are lost. To add your own guidance, write a separate
+  skill or use your tool's project instructions instead.
+-->
+
 Yaak is a desktop API client. The `yaak` CLI reads and writes the **same local
 database as the desktop app**, so anything you create shows up in the app
 immediately, and vice versa. There is no server and no sign-in: `yaak auth` is