fix: restore skills/ and .claude-plugin/ in plugin install tarball (#262)

v3.0.1 added .gitattributes rules that excluded both directories from
git archive output, shrinking the claude.ai .skill bundle. But Claude
Code's /plugin install fetches the SAME archive, so users installing
v3.0.1 or v3.0.2 received a tarball with no plugin manifest and no
skill files. Install appeared successful but the plugin was a useless
empty shell.

Proof:
  git archive v3.0.0 | grep 'skills/|\.claude-plugin/' | wc -l  # 8
  git archive v3.0.1 | grep 'skills/|\.claude-plugin/' | wc -l  # 0
  git archive v3.0.2 | grep 'skills/|\.claude-plugin/' | wc -l  # 0

No issue reports yet because:
 - Cached pre-v3.0.1 installs keep working (it's the new-install path
   that's broken)
 - The breakage is under 24 hours old
 - Users invoking the skill via natural language go through
   skill-selector rather than /last30days slash command

Also reverts v3.0.2's "skills": ["skills"] back to "./", the value
that shipped in every tag from v2.1.0 through v3.0.0. That change was
a misdiagnosis; the manifest wasn't in the tarball anyway so it had
no effect on user-visible installs.

Archive file count after fix: 97 (cap is 200, plenty of room).
Follow-up: move claude.ai-specific bundle exclusions into
scripts/build-skill.sh where they belong, rather than .gitattributes
which cannot distinguish between the two distribution channels.

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>

.gitattributes

@@ -16,9 +16,10 @@ docs/          export-ignore
 fixtures/      export-ignore
 assets/        export-ignore
 
-# Second SKILL.md files would confuse claude.ai's uploader
-# (skills/last30days/ is an internal spec; skills/last30days-nux/ is a symlink)
-skills/        export-ignore
+# NOTE: skills/ and .claude-plugin/ are NOT export-ignored here because
+# Claude Code's /plugin install fetches this same git archive tarball.
+# Removing those from the archive (as v3.0.1 did) silently breaks installs.
+# claude.ai-bundle-specific exclusions live in scripts/build-skill.sh.
 
 # Historical + repo-only manifests
 SKILL-original.md  export-ignore
@@ -35,7 +36,6 @@ uv.lock            export-ignore
 .agents/         export-ignore
 .codex-plugin/   export-ignore
 .hermes-plugin/  export-ignore
-.claude-plugin/  export-ignore
 
 # CI workflows - repo-only, not needed at skill runtime
 .github/         export-ignore