feat: persistent chat history with clear button

- Add dashboard/store.py: MessageLog dataclass singleton tracking
  user/agent/error messages for the lifetime of the server process
- agents.py: write each chat turn to MessageLog; add GET and DELETE
  /agents/timmy/history routes returning the history.html partial
- partials/history.html: render stored messages by role (YOU / TIMMY /
  SYSTEM); falls back to the Mission Control init message when empty
- index.html: chat-log loads history via hx-get on page start; new
  CLEAR button in panel header sends hx-delete to reset the log
- style.css: add .mc-btn-clear (muted, red-on-hover for the header)
- tests: autouse reset_message_log fixture in conftest; 5 new history
  tests covering empty state, recording, offline errors, clear, and
  post-clear state → 32 tests total, all passing

https://claude.ai/code/session_01KZMfwBpLuiv6x9GbzTqbys

static/style.css

@@ -204,6 +204,22 @@ body {
   flex-shrink: 0;
 }
 
+.mc-btn-clear {
+  background: transparent;
+  border: 1px solid var(--border);
+  border-radius: 2px;
+  color: var(--text-dim);
+  font-family: var(--font);
+  font-size: 9px;
+  font-weight: 700;
+  padding: 3px 8px;
+  letter-spacing: 0.12em;
+  cursor: pointer;
+  transition: border-color 0.15s, color 0.15s;
+  touch-action: manipulation;
+}
+.mc-btn-clear:hover { border-color: var(--red); color: var(--red); }
+
 /* Bootstrap form-control overrides */
 .mc-input {
   background: var(--bg-deep) !important;