feat: Sprint 17 -- workspace breadcrumbs, slash commands, send key setting

Track A: Workspace breadcrumb navigation
- Breadcrumb path bar with clickable segments when inside subdirectories
- Up button in panel header for parent directory navigation
- S.currentDir state tracking; file ops stay in current directory
- New file/folder creation respects current subdirectory

Track B: Slash commands foundation
- New commands.js module (7th JS module) with command registry and parser
- Built-in commands: /help, /clear, /model, /workspace, /new
- Autocomplete dropdown on / input with arrow/tab/enter/escape navigation
- Unrecognized commands pass through to agent normally

Track C: Send key setting (closes #26)
- send_key added to settings defaults in api/config.py
- Settings panel dropdown: Enter (default) vs Ctrl/Cmd+Enter
- Keydown handler rewritten for autocomplete + send key preference
- Setting loaded on boot, persisted to settings.json

5 new tests, 242 total (219 passing, 22 pre-existing failures, 0 regressions).

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

static/messages.js

@@ -1,6 +1,10 @@
 async function send(){
   const text=$('msg').value.trim();
   if(!text&&!S.pendingFiles.length)return;
+  // Slash command intercept -- local commands handled without agent round-trip
+  if(text.startsWith('/')&&!S.pendingFiles.length&&executeCommand(text)){
+    $('msg').value='';autoResize();hideCmdDropdown();return;
+  }
   // Don't send while an inline message edit is active
   if(document.querySelector('.msg-edit-area'))return;
   // If busy, queue the message instead of dropping it