docs: name the skill directory in the skills-only install commands

docs/installation.md:101 and docs/windows.md:38 copy skills/* into the user's
skills directory. Since the v3.11.0 move of the language variants to
skills/i18n/, that glob also copies i18n/, which is not a skill: the user ends
up with ~/.claude/skills/i18n/ holding no SKILL.md, and the five translations
at ~/.claude/skills/i18n/planning-with-files-<lang>/ rather than the
~/.claude/skills/planning-with-files-<lang>/ that commands/plan-<lang>.md
probes first.

Both now name skills/planning-with-files, matching section 4 of
installation.md, README.md:250 and every other copy command in docs/. A second
line copying skills/i18n/* would restore what the glob used to install, but
docs/languages.md:26 states that installing English installs no translation,
and six skills in ~/.claude/skills/ returns the five extra descriptions to
every session that PR #226 removed from the plugin route.

The guard lives in tests/test_plugin_skill_surface.py, which owns the skills/
depth invariant. It matches only lines starting with cp or Copy-Item, so the
prose describing the skills/*/SKILL.md plugin scan does not trip it.