feat: Sprint 23 — agentic transparency + polish

Track A: Token/cost display
- Read agent usage attrs (session_prompt_tokens, session_completion_tokens,
  session_estimated_cost_usd) after run_conversation in streaming.py
- Add input_tokens, output_tokens, estimated_cost fields to Session model
- Include usage in done SSE event payload
- Store usage on S.lastUsage in messages.js done handler
- Render usage badge below last assistant message (input/output/cost)

Track B: Subagent delegation cards
- Add subagent_progress to toolIcon map with shuffle emoji
- Special-case subagent_progress in buildToolCard: "Subagent" label,
  strip double emoji from preview, add tool-card-subagent CSS class
- Indented border-left styling for subagent cards
- Clean delegate_task display name

Track C: Skill picker in cron create form
- Add skill search input + tag chips to cron create form HTML
- Skill picker JS in panels.js: search/filter, click-to-add tags,
  remove tag chips, pre-fetch skill list on form open
- submitCronCreate sends skills array in POST body
- Skill picker dropdown + tag CSS

Track D: Skill linked files viewer
- Add file query param to /api/skills/content endpoint
- Serve linked files from skill directory with path traversal protection
- Ensure linked_files key always present in skill content response
- Render linked files section below SKILL.md content in preview panel
- openSkillFile function for viewing individual linked files

Track E: Bug fixes and code quality
- Expand Session.__init__ and compact() to readable multi-line format
- Remove inline import json as _j2 inside loop in streaming.py
- Fix tool_calls: capture args from assistant messages, skip unresolved names
- Store args snapshot in persisted tool_calls for reload display

6 new tests. Total: 421 (409 passing).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/routes.py

@@ -223,11 +223,26 @@ def handle_get(handler, parsed):
         return j(handler, {'skills': data.get('skills', [])})
 
     if parsed.path == '/api/skills/content':
-        from tools.skills_tool import skill_view as _skill_view
-        name = parse_qs(parsed.query).get('name', [''])[0]
+        from tools.skills_tool import skill_view as _skill_view, SKILLS_DIR
+        qs = parse_qs(parsed.query)
+        name = qs.get('name', [''])[0]
         if not name: return j(handler, {'error': 'name required'}, status=400)
+        file_path = qs.get('file', [''])[0]
+        if file_path:
+            # Serve a linked file from the skill directory
+            skill_dir = None
+            for p in SKILLS_DIR.rglob(name):
+                if p.is_dir(): skill_dir = p; break
+            if not skill_dir: return bad(handler, 'Skill not found', 404)
+            target = (skill_dir / file_path).resolve()
+            try: target.relative_to(skill_dir.resolve())
+            except ValueError: return bad(handler, 'Invalid file path', 400)
+            if not target.exists() or not target.is_file():
+                return bad(handler, 'File not found', 404)
+            return j(handler, {'content': target.read_text(encoding='utf-8'), 'path': file_path})
         raw = _skill_view(name)
         data = json.loads(raw) if isinstance(raw, str) else raw
+        if 'linked_files' not in data: data['linked_files'] = {}
         return j(handler, data)
 
     # ── Memory API (GET) ──