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.

src/web/routes/session-routes.ts

@@ -406,6 +406,7 @@ export function registerSessionRoutes(
               ? modelConfig?.defaultModel || undefined
               : undefined;
     const claudeModeConfig = await ctx.getClaudeModeConfig();
+    const terminalHistoryConfig = await ctx.getTerminalHistoryConfig();
     const session = new Session({
       workingDir,
       mode,
@@ -422,6 +423,7 @@ export function registerSessionRoutes(
       resumeSessionId: validatedResumeId,
       envOverrides: body.envOverrides,
       effort: body.effort,
+      tmuxHistoryLimit: terminalHistoryConfig.tmuxHistoryLimit,
     });
 
     ctx.addSession(session);
@@ -1362,6 +1364,7 @@ export function registerSessionRoutes(
               ? qsModelConfig?.defaultModel || undefined
               : undefined;
     const qsClaudeModeConfig = await ctx.getClaudeModeConfig();
+    const qsTerminalHistoryConfig = await ctx.getTerminalHistoryConfig();
     const session = new Session({
       workingDir: casePath,
       mux: ctx.mux,
@@ -1376,6 +1379,7 @@ export function registerSessionRoutes(
       geminiConfig: mode === 'gemini' ? geminiConfig : undefined,
       envOverrides,
       effort,
+      tmuxHistoryLimit: qsTerminalHistoryConfig.tmuxHistoryLimit,
     });
 
     // Auto-detect completion phrase from CLAUDE.md BEFORE broadcasting

src/web/routes/system-routes.ts

@@ -49,6 +49,7 @@ import type { SessionPort, EventPort, ConfigPort, InfraPort, AuthPort } from '..
 import { AUTH_COOKIE_NAME } from '../middleware/auth.js';
 import { QR_AUTH_FAILURE_MAX } from '../../config/tunnel-config.js';
 import { AUTH_SESSION_TTL_MS } from '../../config/auth-config.js';
+import { resolveTerminalHistoryConfig } from '../../config/terminal-history.js';
 
 // Maximum screenshot upload size (10MB)
 const MAX_SCREENSHOT_SIZE = 10 * 1024 * 1024;
@@ -624,6 +625,11 @@ export function registerSystemRoutes(
       const merged = { ...existing, ...settingsToStore };
       await fs.writeFile(SETTINGS_PATH, JSON.stringify(merged, null, 2));
 
+      // Apply a changed tmux history-limit to all live sessions immediately.
+      if (settings.tmuxHistoryLimit !== undefined) {
+        await ctx.mux.setHistoryLimit(resolveTerminalHistoryConfig(merged).tmuxHistoryLimit);
+      }
+
       // Handle subagent tracking toggle dynamically
       toggleService((settings.subagentTrackingEnabled as boolean) ?? true, subagentWatcher, 'Subagent watcher');