COD-80 raise terminal history/scrollback/buffer defaults

Bump the centralized terminal-history defaults: tmux scrollback 50k->100k and
PTY buffer cap 2MB->32MB (trim 1.5MB->24MB). Both remain env/settings overridable
and bounds-clamped. Worst-case 20-session buffer budget rises 40MB->640MB.
Stacked on the terminal-history config commit.

src/config/buffer-limits.ts

@@ -6,10 +6,9 @@
  * it easy to tune memory usage.
  *
  * Memory Budget Rationale (for 20 concurrent sessions):
- * - Terminal buffer: 2MB max × 20 = 40MB worst case
+ * - Terminal buffer: 32MB max × 20 = 640MB worst case
  * - Text output: 1MB max × 20 = 20MB worst case
  * - Messages: ~1KB each × 1000 × 20 = 20MB worst case
- * - Total buffer overhead: ~80MB (acceptable for a long-running server)
  *
  * @module config/buffer-limits
  */