COD-80 centralize terminal history/scrollback/buffer limits into config

Introduce src/config/terminal-history.ts: one place for terminal scrollback,
tmux history-limit, and PTY buffer byte caps, each overridable via env var or
the settings object and bounds-clamped via resolveTerminalHistoryConfig().
Defaults match the prior hardcoded values, so this is behavior-neutral. Wires
the resolver through buffer-limits, tmux-manager (incl. a setHistoryLimit so a
settings change applies live), session, server, system-routes, session-routes,
schemas, and the config port. Adds 4 optional settings keys (terminalScrollback
Lines, tmuxHistoryLimit, terminalBufferMaxBytes, terminalBufferTrimBytes) with
bounds + a trim<=max cross-check.

test/mocks/mock-route-context.ts

@@ -9,6 +9,7 @@
  */
 import { vi } from 'vitest';
 import { MockSession, createMockSession } from './mock-session.js';
+import { resolveTerminalHistoryConfig } from '../../src/config/terminal-history.js';
 
 /**
  * Creates a mock context that satisfies all port interfaces.
@@ -76,6 +77,7 @@ export function createMockRouteContext(options?: { sessionId?: string }) {
     getGlobalNiceConfig: vi.fn(async () => undefined),
     getModelConfig: vi.fn(async () => null),
     getClaudeModeConfig: vi.fn(async () => ({})),
+    getTerminalHistoryConfig: vi.fn(async () => resolveTerminalHistoryConfig({})),
     getDefaultClaudeMdPath: vi.fn(async () => undefined),
     getLightState: vi.fn(() => ({ sessions: [], status: 'ok' })),
     getLightSessionsState: vi.fn(() => {
@@ -99,6 +101,7 @@ export function createMockRouteContext(options?: { sessionId?: string }) {
       getSession: vi.fn(() => null),
       clearRespawnConfig: vi.fn(),
       updateRespawnConfig: vi.fn(),
+      setHistoryLimit: vi.fn(async () => {}),
     },
     runSummaryTrackers: new Map(),
     activePlanOrchestrators: new Map(),