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/style.css

@@ -562,6 +562,26 @@ body.resizing{user-select:none;cursor:col-resize;}
 /* Show more button inside tool card result */
 .tool-card-more{background:none;border:none;color:var(--blue);font-size:10px;cursor:pointer;padding:3px 0 0;opacity:.7;display:block;}
 .tool-card-more:hover{opacity:1;}
+/* Subagent cards: indented with accent border */
+.tool-card-subagent{border-left:2px solid rgba(124,185,255,.3);margin-left:8px;}
+/* Token usage badge below assistant messages */
+.msg-usage{font-size:11px;color:var(--muted);opacity:.6;margin-top:2px;padding-left:42px;}
+.msg-usage:hover{opacity:1;}
+/* Skill picker (cron create form) */
+.skill-picker-wrap{position:relative;}
+.skill-picker-dropdown{position:absolute;left:0;right:0;top:100%;background:var(--sidebar);border:1px solid var(--border2);border-radius:6px;z-index:10;max-height:180px;overflow-y:auto;box-shadow:0 4px 12px rgba(0,0,0,.3);}
+.skill-opt{padding:6px 10px;cursor:pointer;font-size:12px;color:var(--muted);transition:background .1s;}
+.skill-opt:hover{background:rgba(255,255,255,.08);color:var(--text);}
+.skill-picker-tags{display:flex;flex-wrap:wrap;gap:4px;margin-top:4px;}
+.skill-tag{background:rgba(124,185,255,.12);border:1px solid rgba(124,185,255,.25);border-radius:12px;padding:2px 8px;font-size:11px;color:var(--blue);display:flex;align-items:center;gap:4px;}
+.remove-tag{cursor:pointer;opacity:.6;font-size:13px;line-height:1;}
+.remove-tag:hover{opacity:1;color:var(--accent);}
+/* Skill linked files section */
+.skill-linked-files{margin-top:16px;border-top:1px solid var(--border);padding-top:12px;}
+.skill-linked-section{margin-bottom:8px;}
+.skill-linked-section h4{font-size:10px;text-transform:uppercase;letter-spacing:.05em;color:var(--muted);margin-bottom:4px;}
+.skill-linked-file{display:block;font-size:12px;padding:3px 6px;border-radius:4px;cursor:pointer;color:var(--blue);text-decoration:none;}
+.skill-linked-file:hover{background:rgba(255,255,255,.06);}
 .tool-card-row{margin:0;padding:1px 0;}
 .tool-card{background:rgba(255,255,255,.03);border:1px solid rgba(255,255,255,.07);border-radius:6px;margin:2px 0 2px 40px;overflow:hidden;transition:border-color .15s;}
 .tool-card:hover{border-color:rgba(255,255,255,.12);}