refactor: consolidate ~70 scattered constants into 6 new config files (phase 6)

Create 6 domain-focused config files in src/config/ to centralize operational
tuning knobs that were scattered across 15+ source files:

- server-timing.ts: SSE batching, state debounce, scheduled runs, error recovery
- auth-config.ts: session TTL, rate limits, hook timeout
- tunnel-config.ts: QR token rotation, tunnel process lifecycle
- terminal-limits.ts: input length, terminal cols/rows, session name
- ai-defaults.ts: AI model string, idle/plan context limits
- team-config.ts: poll interval, cache sizes

Eliminates all cross-file duplicates:
- AI model string: 5 occurrences → 1 (config only)
- STATS_COLLECTION_INTERVAL_MS: 2 → 1
- timeout: 10000 in hooks-config: 6 → 0 (now HOOK_TIMEOUT_MS)
- MAX_TRACKED_AGENTS shadow: removed, imports from map-limits

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/config/terminal-limits.ts

@@ -0,0 +1,21 @@
+/**
+ * @fileoverview Terminal dimension and input validation limits.
+ *
+ * Used by API routes to validate resize, input, and session
+ * creation requests. Separate from buffer-limits.ts which
+ * controls memory buffer sizes.
+ *
+ * @module config/terminal-limits
+ */
+
+/** Max input length per API request (bytes) */
+export const MAX_INPUT_LENGTH = 64 * 1024;
+
+/** Max terminal columns for resize requests */
+export const MAX_TERMINAL_COLS = 500;
+
+/** Max terminal rows for resize requests */
+export const MAX_TERMINAL_ROWS = 200;
+
+/** Max session name length (chars) */
+export const MAX_SESSION_NAME_LENGTH = 128;