refactor: implement three-layer agent architecture (agents / prompts / skills)

Layer 1 — src/agents/ (thin agent definitions, no prompt text)
  registry.ts   — AgentConfig, registerAgent(), getAgent(), AGENTS proxy, pick()
  orchestrator.ts, coder.ts, pm.ts, marketing.ts — one file each, just metadata + tool picks
  index.ts      — barrel: imports prompts then agents (correct registration order)

Layer 2 — src/prompts/ (prompt text separated from agent logic)
  loader.ts     — registerPrompt(), resolvePrompt() with {{variable}} substitution
  orchestrator.ts, coder.ts, pm.ts, marketing.ts — prompt templates as registered strings
  orchestrator.ts now uses resolvePrompt('orchestrator', { knowledge }) instead of
  inline SYSTEM_PROMPT const; {{knowledge}} variable injects project memory cleanly.
  agent-runner.ts uses resolvePrompt(config.promptId) per agent turn.

Layer 3 — src/tools/skills.ts (new skills capability)
  list_skills(repo)      — lists .skills/<name>/SKILL.md directories from a Gitea repo
  get_skill(repo, name)  — reads and returns the markdown body of a skill file
  Orchestrator and all agents now have get_skill in their tool sets.
  Orchestrator also has list_skills and references skills in its prompt.

Also fixed:
  - server.ts now passes history + knowledge_context from request body to orchestratorChat()
    (these were being sent by the frontend but silently dropped)
  - server.ts imports PROTECTED_GITEA_REPOS from tools/security.ts (no more duplicate)
  - Deleted src/agents.ts (replaced by src/agents/ directory)

Made-with: Cursor

dist/agents/registry.d.ts

@@ -0,0 +1,18 @@
+import { ToolDefinition } from '../tools';
+export interface AgentConfig {
+    name: string;
+    description: string;
+    model: string;
+    promptId: string;
+    tools: ToolDefinition[];
+}
+export declare function registerAgent(config: AgentConfig): void;
+export declare function getAgent(name: string): AgentConfig | undefined;
+export declare function allAgents(): AgentConfig[];
+/**
+ * Backwards-compatible AGENTS object — populated as agents register.
+ * server.ts uses AGENTS[name] and Object.values(AGENTS).
+ */
+export declare const AGENTS: Record<string, AgentConfig>;
+/** Pick tools from ALL_TOOLS by name. */
+export declare function pick(names: string[]): ToolDefinition[];