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>

static/index.html

@@ -45,11 +45,16 @@
         <input id="cronFormName" placeholder="Job name (optional)" style="width:100%;background:rgba(255,255,255,.05);border:1px solid var(--border2);border-radius:6px;color:var(--text);padding:5px 8px;font-size:12px;outline:none;margin-bottom:6px">
         <input id="cronFormSchedule" placeholder="Schedule: '0 9 * * *' or 'every 1h'" style="width:100%;background:rgba(255,255,255,.05);border:1px solid var(--border2);border-radius:6px;color:var(--text);padding:5px 8px;font-size:12px;outline:none;margin-bottom:6px">
         <textarea id="cronFormPrompt" rows="3" placeholder="Prompt (must be self-contained)" style="width:100%;background:rgba(255,255,255,.05);border:1px solid var(--border2);border-radius:6px;color:var(--text);padding:5px 8px;font-size:12px;outline:none;resize:none;font-family:inherit;margin-bottom:6px"></textarea>
-        <select id="cronFormDeliver" style="width:100%;background:rgba(255,255,255,.05);border:1px solid var(--border2);border-radius:6px;color:var(--text);padding:5px 8px;font-size:12px;outline:none;margin-bottom:8px">
+        <select id="cronFormDeliver" style="width:100%;background:rgba(255,255,255,.05);border:1px solid var(--border2);border-radius:6px;color:var(--text);padding:5px 8px;font-size:12px;outline:none;margin-bottom:6px">
           <option value="local">Local (save output only)</option>
           <option value="discord">Discord</option>
           <option value="telegram">Telegram</option>
         </select>
+        <div class="skill-picker-wrap" style="margin-bottom:8px">
+          <input id="cronFormSkillSearch" placeholder="Add skills (optional)..." style="width:100%;background:rgba(255,255,255,.05);border:1px solid var(--border2);border-radius:6px;color:var(--text);padding:5px 8px;font-size:12px;outline:none" autocomplete="off">
+          <div id="cronFormSkillDropdown" class="skill-picker-dropdown" style="display:none"></div>
+          <div id="cronFormSkillTags" class="skill-picker-tags"></div>
+        </div>
         <div style="display:flex;gap:6px">
           <button class="cron-btn run" style="flex:1" onclick="submitCronCreate()">Create job</button>
           <button class="cron-btn" style="flex:1" onclick="toggleCronForm()">Cancel</button>