feat: add P2 features — retry policy, prompt templating, usage tracking, tech debt cleanup

- Extract shared splitMessage() into channels/utils.ts (dedup 4 adapters)
- Add Slack user name resolution with caching (users.info API)
- Add withRetry() with exponential backoff + jitter, isRetryable() filter
- Wire retry config into ModelRouter.chat() (non-streaming only)
- Add assembleSystemPrompt() multi-file template system (SOUL/AGENTS/IDENTITY/USER/TOOLS.md)
- Add usage tracking accumulators in NativeAgent + AgentOrchestrator
- Add estimateCost() with per-model pricing table
- Add /usage TUI command with full usage report formatting
- Add retrySchema and promptSchema to config schema

Tests: 569 passing, typecheck clean

src/config/schema.ts

@@ -146,6 +146,14 @@ const processSchema = z.object({
   buffer_size: z.number().min(1024).max(1048576).default(65536),
 }).default({});
 
+const retrySchema = z.object({
+  enabled: z.boolean().default(true),
+  max_retries: z.number().min(0).max(10).default(3),
+  initial_delay_ms: z.number().min(100).max(60000).default(1000),
+  backoff_multiplier: z.number().min(1).max(5).default(2),
+  max_delay_ms: z.number().min(1000).max(120000).default(30000),
+}).default({});
+
 const webSearchSchema = z.object({
   provider: z.enum(['brave', 'searxng']).default('brave'),
   api_key: z.string().optional(),
@@ -153,6 +161,16 @@ const webSearchSchema = z.object({
   max_results: z.number().min(1).max(20).default(5),
 }).default({});
 
+const promptSchema = z.object({
+  /** Additional directories to search for prompt template files. */
+  search_dirs: z.array(z.string()).default([]),
+  /** Extra named sections to include in the system prompt. */
+  extra_sections: z.array(z.object({
+    name: z.string(),
+    content: z.string(),
+  })).default([]),
+}).default({});
+
 export const configSchema = z.object({
   telegram: telegramSchema,
   discord: discordSchema,
@@ -169,7 +187,9 @@ export const configSchema = z.object({
   compaction: compactionSchema,
   memory: memorySchema,
   process: processSchema,
+  retry: retrySchema,
   web_search: webSearchSchema,
+  prompt: promptSchema,
 });
 
 export type Config = z.infer<typeof configSchema>;
@@ -184,3 +204,5 @@ export type ProcessConfig = z.infer<typeof processSchema>;
 export type DiscordConfig = z.infer<typeof discordSchema>;
 export type SlackConfig = z.infer<typeof slackSchema>;
 export type WhatsAppConfig = z.infer<typeof whatsappSchema>;
+export type RetryPolicyConfig = z.infer<typeof retrySchema>;
+export type PromptConfig = z.infer<typeof promptSchema>;