install.sh: drop .skill-source marker in deployed skill dir

The skill directory at ~/.agents/skills/opencode-mempalace-bridge/ is a
real dir containing a single SKILL.md symlink back into this repo — the
'colocated skill' pattern. Sibling reconcilers (skillset's
deploy-skills.sh, cli_utils's agents-sync.zsh) already handle external
dirs correctly via their existing 'leave real dirs alone' policies, but
a machine-readable marker makes ownership explicit:

  # skill-source: mempalace-toolkit
  # repo: <absolute path>
  # url: ssh://git@gitea.jordbo.se:2222/joakimp/mempalace-toolkit.git

The marker is the convention for any external repo that wants to ship a
colocated skill. The name is generic (.skill-source, not
.managed-by-mempalace-toolkit) so a second colocated skill from a
different repo can reuse the same file name; the first line identifies
the owner.

--uninstall now also removes the marker (only if it still says
mempalace-toolkit) and the now-empty skill dir.

AGENTS.md + README.md describe the pattern and point at sibling docs in
cli_utils/AGENTS-SYNC.md and skillset/README.md that mirror the
convention.

install.sh

@@ -38,10 +38,13 @@ What install does:
   - Symlinks SKILL.md into ~/.agents/skills/opencode-mempalace-bridge/SKILL.md
     (auto-discovered by opencode; run agents-sync from cli_utils to also
     reach Claude Code and Kiro)
+  - Drops a .skill-source marker in the skill dir so sibling tooling
+    (deploy-skills.sh, agents-sync.zsh) knows the dir is externally owned
 
 What uninstall does:
   - Removes symlinks in ~/.local/bin/ that point into this repo
   - Removes the skill symlink if it points into this repo
+  - Removes the .skill-source marker and empty skill dir
 EOF
             exit 0 ;;
         *) echo "Unknown flag: $1" >&2; exit 2 ;;
@@ -98,6 +101,26 @@ install_bin() {
 install_skill() {
     note "Linking companion agent skill"
     mkdir -p "$SKILL_DEST_DIR"
+
+    # Drop a marker file so sibling tooling (deploy-skills.sh, agents-sync.zsh,
+    # and any future reconciler) can tell at a glance that this skill directory
+    # is owned by an external repo and shouldn't be clobbered. The convention
+    # is two lines: "# skill-source: <repo-name>" + "# url: <clone-url>".
+    # Any colocated skill from any repo can adopt the same convention.
+    local marker="$SKILL_DEST_DIR/.skill-source"
+    if [[ ! -e "$marker" ]]; then
+        cat > "$marker" <<EOF
+# skill-source: mempalace-toolkit
+# repo: $SCRIPT_DIR
+# url: ssh://git@gitea.jordbo.se:2222/joakimp/mempalace-toolkit.git
+# This skill directory is managed by an external repo's install.sh.
+# deploy-skills.sh (skillset) and agents-sync.zsh (cli_utils) leave
+# directories containing this marker alone. Do not move SKILL.md out
+# of this directory without also updating the owning repo.
+EOF
+        ok "Wrote $marker"
+    fi
+
     if [[ -e "$SKILL_DEST" || -L "$SKILL_DEST" ]]; then
         if link_if_into_repo "$SKILL_DEST"; then
             ok "Skill already linked"
@@ -174,6 +197,18 @@ do_uninstall() {
         ok "No skill symlink to remove"
     fi
 
+    # Remove the marker and the now-empty skill directory, but only if
+    # the marker was written by us and the directory has nothing else in it.
+    local marker="$SKILL_DEST_DIR/.skill-source"
+    if [[ -f "$marker" ]] && grep -q '^# skill-source: mempalace-toolkit$' "$marker" 2>/dev/null; then
+        rm "$marker"
+        ok "Removed $marker"
+    fi
+    if [[ -d "$SKILL_DEST_DIR" ]] && [[ -z "$(ls -A "$SKILL_DEST_DIR" 2>/dev/null)" ]]; then
+        rmdir "$SKILL_DEST_DIR"
+        ok "Removed empty $SKILL_DEST_DIR"
+    fi
+
     echo
     ok "Done."
 }