Add BRAIN_DISABLED flag and fix Ollama tool call formatting

Features:
  - Add BRAIN_DISABLED feature flag to hide all Brain functionality
  - When enabled, hides Brain banner, status indicator, menu, and commands
  - Flag location: src/constants/brain.ts

  Fixes:
  - Fix Ollama 400 error by properly formatting tool_calls in messages
  - Update OllamaMessage type to include tool_calls field
  - Fix Brain menu keyboard not working (add missing modes to isMenuOpen)

  UI Changes:
  - Remove "^Tab toggle mode" hint from status bar
  - Remove "ctrl+t to hide todos" hint from status bar

  Files modified:
  - src/constants/brain.ts (add BRAIN_DISABLED flag)
  - src/types/ollama.ts (add tool_calls to OllamaMessage)
  - src/providers/ollama/chat.ts (format tool_calls in messages)
  - src/tui-solid/components/header.tsx (hide Brain UI when disabled)
  - src/tui-solid/components/status-bar.tsx (remove hints)
  - src/tui-solid/components/command-menu.tsx (filter brain command)
  - src/tui-solid/components/input-area.tsx (fix isMenuOpen modes)
  - src/tui-solid/routes/session.tsx (skip brain menu when disabled)
  - src/services/brain.ts (early return when disabled)
  - src/services/chat-tui/initialize.ts (skip brain init when disabled)

src/types/agent-definition.ts

@@ -0,0 +1,91 @@
+/**
+ * Agent markdown definition types
+ * Agents are defined in markdown files with YAML frontmatter
+ * Location: .codetyper/agents/*.md
+ */
+
+export type AgentTier = "fast" | "balanced" | "thorough";
+
+export type AgentColor =
+  | "red"
+  | "green"
+  | "blue"
+  | "yellow"
+  | "cyan"
+  | "magenta"
+  | "white"
+  | "gray";
+
+export interface AgentDefinition {
+  readonly name: string;
+  readonly description: string;
+  readonly tools: ReadonlyArray<string>;
+  readonly tier: AgentTier;
+  readonly color: AgentColor;
+  readonly maxTurns?: number;
+  readonly systemPrompt?: string;
+  readonly triggerPhrases?: ReadonlyArray<string>;
+  readonly capabilities?: ReadonlyArray<string>;
+  readonly permissions?: AgentPermissions;
+}
+
+export interface AgentPermissions {
+  readonly allowedPaths?: ReadonlyArray<string>;
+  readonly deniedPaths?: ReadonlyArray<string>;
+  readonly allowedTools?: ReadonlyArray<string>;
+  readonly deniedTools?: ReadonlyArray<string>;
+  readonly requireApproval?: ReadonlyArray<string>;
+}
+
+export interface AgentDefinitionFile {
+  readonly filePath: string;
+  readonly frontmatter: AgentFrontmatter;
+  readonly content: string;
+  readonly parsed: AgentDefinition;
+}
+
+export interface AgentFrontmatter {
+  readonly name: string;
+  readonly description: string;
+  readonly tools: ReadonlyArray<string>;
+  readonly tier?: AgentTier;
+  readonly color?: AgentColor;
+  readonly maxTurns?: number;
+  readonly triggerPhrases?: ReadonlyArray<string>;
+  readonly capabilities?: ReadonlyArray<string>;
+  readonly allowedPaths?: ReadonlyArray<string>;
+  readonly deniedPaths?: ReadonlyArray<string>;
+}
+
+export interface AgentRegistry {
+  readonly agents: ReadonlyMap<string, AgentDefinition>;
+  readonly byTrigger: ReadonlyMap<string, string>;
+  readonly byCapability: ReadonlyMap<string, ReadonlyArray<string>>;
+}
+
+export interface AgentLoadResult {
+  readonly success: boolean;
+  readonly agent?: AgentDefinition;
+  readonly error?: string;
+  readonly filePath: string;
+}
+
+export const DEFAULT_AGENT_DEFINITION: Partial<AgentDefinition> = {
+  tier: "balanced",
+  color: "cyan",
+  maxTurns: 10,
+  tools: ["read", "glob", "grep"],
+  capabilities: [],
+  triggerPhrases: [],
+};
+
+export const AGENT_TIER_MODELS: Record<AgentTier, string> = {
+  fast: "gpt-4o-mini",
+  balanced: "gpt-4o",
+  thorough: "o1",
+};
+
+export const AGENT_DEFINITION_SCHEMA = {
+  required: ["name", "description", "tools"],
+  optional: ["tier", "color", "maxTurns", "triggerPhrases", "capabilities", "allowedPaths", "deniedPaths"],
+};