fix(skills): tell the model where each skill body lives

The system-prompt addendum now tags every skill with a source
pointer: '[builtin]' for skills embedded in the zot binary, or the
SKILL.md path (HOME collapsed to ~) for user-installed ones. The
body still loads on demand through the 'skill' tool by name, so
no behaviour change for execution, this is pure disambiguation.

Before:
  - write-zot-extension — Help the user create a new zot extension...

After:
  - write-zot-extension [builtin]: Help the user create a new zot extension...
  - code-review [~/Library/Application Support/zot/skills/code-review/SKILL.md]: ...

Why: built-in skills have no filesystem path because their
markdown is embedded in the binary. Without the [builtin] tag the
model had no way to distinguish them from user skills, and could
mistakenly try to read a nonexistent file. The path pointer for
user skills also helps the model cite where guidance came from
and reason about trust (builtin vs project-local vs global).

Test updated; addendum grew to ~123 tokens for a typical 3-skill
setup (code-review, test-fix, write-zot-extension).

internal/skills/skills.go

@@ -155,24 +155,56 @@ func scanUserSkills(zotHome, cwd, userHome string, seen map[string]*Skill) []err
 // SystemPromptAddendum returns the text to append to the system
 // prompt when at least one skill is loaded. Empty string if none.
 //
-// Format kept short and explicit so the model reliably calls the
-// `skill` tool with a name from the list rather than guessing.
+// The format is deliberately compact: name, one-line description,
+// and a source pointer telling the model where the full body
+// lives. Built-in skills show "builtin" since their markdown is
+// embedded in the zot binary and not on the filesystem; user
+// skills show their SKILL.md path (shortened with ~ for HOME).
+//
+// Loading still goes through the `skill` tool with just the name.
+// The pointer is there so the model can (a) mention the source
+// honestly in explanations and (b) distinguish between built-ins
+// and user-authored instruction sets when reasoning about trust.
 func SystemPromptAddendum(skills []*Skill) string {
 	if len(skills) == 0 {
 		return ""
 	}
+	home, _ := os.UserHomeDir()
 	var sb strings.Builder
-	sb.WriteString("Available skills (call the `skill` tool with one of these names to load full instructions):\n")
+	sb.WriteString("Available skills (call the `skill` tool with a name from this list to load its full instructions):\n")
 	for _, s := range skills {
 		desc := strings.TrimSpace(s.Description)
 		if desc == "" {
 			desc = "(no description)"
 		}
-		fmt.Fprintf(&sb, "- %s — %s\n", s.Name, desc)
+		pointer := skillSourcePointer(s, home)
+		fmt.Fprintf(&sb, "- %s [%s]: %s\n", s.Name, pointer, desc)
 	}
 	return sb.String()
 }
 
+// skillSourcePointer returns a short tag describing where a skill
+// originates. Built-ins are tagged "builtin" because their markdown
+// is embedded in the zot binary and not reachable through the
+// filesystem. User skills are tagged with their SKILL.md path,
+// collapsed to use ~ for the user home when possible.
+func skillSourcePointer(s *Skill, home string) string {
+	if s == nil {
+		return "unknown"
+	}
+	if s.Builtin {
+		return "builtin"
+	}
+	p := s.Path
+	if p == "" {
+		return "unknown"
+	}
+	if home != "" && strings.HasPrefix(p, home+string(filepath.Separator)) {
+		return "~" + p[len(home):]
+	}
+	return p
+}
+
 // FindByName returns the skill with the given name, or nil.
 func FindByName(skills []*Skill, name string) *Skill {
 	for _, s := range skills {

internal/skills/skills_test.go

@@ -108,12 +108,15 @@ func TestVisibleSkillsHidesBuiltins(t *testing.T) {
 
 func TestSystemPromptAddendum(t *testing.T) {
 	skills := []*Skill{
-		{Name: "a", Description: "Do A."},
-		{Name: "b", Description: "Do B."},
+		{Name: "built-a", Description: "Do A.", Builtin: true},
+		{Name: "user-b", Description: "Do B.", Path: "/tmp/skills/user-b/SKILL.md"},
 	}
 	out := SystemPromptAddendum(skills)
-	if want := "- a — Do A.\n- b — Do B.\n"; !contains(out, want) {
-		t.Errorf("addendum missing entries:\n%s", out)
+	if !contains(out, "- built-a [builtin]: Do A.\n") {
+		t.Errorf("builtin entry missing or wrong:\n%s", out)
+	}
+	if !contains(out, "- user-b [/tmp/skills/user-b/SKILL.md]: Do B.\n") {
+		t.Errorf("user entry missing path pointer:\n%s", out)
 	}
 }