flatten: skills directly at repo root, no skills/ subdir

~/dev/skills/skills/boss/ was visually awkward (skills/skills
read twice) and structurally unnecessary — the skills ARE the
repo's main content, not a sub-collection inside it.

Layout changes:
- boss/SKILL.md moves to the repo root
- skills/README.md (migration notes) moves to docs/migration.md
  where documentation-about-the-system belongs
- install.sh discovers skills via the SKILL.md marker file
  instead of a hardcoded skills/<name>/ path, so future skill
  additions just drop in at the root

Cross-references in boss/SKILL.md (../README.md, ../brainstorm,
etc.) now resolve against the repo root instead of the old
skills/ subdir, which is what they describe: top-level
README is the skill table; sibling skill dirs are the other
migrated skills.

install.sh

@@ -1,9 +1,10 @@
 #!/usr/bin/env bash
 # Install the skills plugin globally into ~/.claude/.
 #
-# For each skill <name> under skills/<name>/, creates symlinks:
-#   ~/.claude/skills/<name>  -> <repo>/skills/<name>
-#   ~/.claude/agents/<name>  -> <repo>/skills/<name>/agents   (if that dir exists)
+# Each top-level directory that contains a SKILL.md is treated as
+# a skill. For each such <name>, creates symlinks:
+#   ~/.claude/skills/<name>  -> <repo>/<name>
+#   ~/.claude/agents/<name>  -> <repo>/<name>/agents   (if that dir exists)
 #
 # Idempotent — existing symlinks pointing to this repo are left
 # alone; existing entries that point elsewhere are reported and
@@ -39,16 +40,15 @@ link_one() {
     echo "link  $dst -> $src"
 }
 
-if [ -d "$REPO_DIR/skills" ]; then
-    for s in "$REPO_DIR"/skills/*/; do
-        [ -d "$s" ] || continue
-        name="$(basename "$s")"
-        link_one "$s" "$CLAUDE_SKILLS/$name"
-        if [ -d "$s/agents" ]; then
-            link_one "$s/agents" "$CLAUDE_AGENTS/$name"
-        fi
-    done
-fi
+for d in "$REPO_DIR"/*/; do
+    [ -d "$d" ] || continue
+    [ -f "$d/SKILL.md" ] || continue
+    name="$(basename "$d")"
+    link_one "${d%/}" "$CLAUDE_SKILLS/$name"
+    if [ -d "$d/agents" ]; then
+        link_one "${d%/}/agents" "$CLAUDE_AGENTS/$name"
+    fi
+done
 
 echo
 echo "Install complete."