refactor(extensions): move cmux into sf extension as internal module

cmux was a standalone extension directory with no extension-manifest.json,
functioning as a utility library for the sf extension. Moving it into sf/cmux/
makes the dependency explicit and removes the orphaned extension directory.

Import paths updated:
- commands-cmux.js, notifications.js, auto.js: ../cmux → ./cmux
- bootstrap/system-context.js: ../../cmux → ../cmux
- subagent/index.js: ../../cmux → ../cmux

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/resources/extensions/sf/auto.js

@@ -33,7 +33,7 @@ import {
 	clearCmuxSidebar,
 	logCmuxEvent,
 	syncCmuxSidebar,
-} from "../cmux/index.js";
+} from "./cmux/index.js";
 import { collectSecretsFromManifest } from "../get-secrets-from-user.js";
 import { getRtkSessionSavings } from "../shared/rtk-session-stats.js";
 import { deactivateSF } from "../shared/sf-phase-state.js";

src/resources/extensions/sf/bootstrap/system-context.js

@@ -4,7 +4,7 @@ import { join } from "node:path";
 import {
 	markCmuxPromptShown,
 	shouldPromptToEnableCmux,
-} from "../../cmux/index.js";
+} from "../cmux/index.js";
 import { toPosixPath } from "../../shared/mod.js";
 import { isCanAskUser } from "../auto.js";
 import { getActiveAutoWorktreeContext } from "../auto-worktree.js";

src/resources/extensions/sf/commands-cmux.js

@@ -4,7 +4,7 @@ import {
 	clearCmuxSidebar,
 	detectCmuxEnvironment,
 	resolveCmuxConfig,
-} from "../cmux/index.js";
+} from "./cmux/index.js";
 import {
 	ensurePreferencesFile,
 	serializePreferencesToFrontmatter,

src/resources/extensions/sf/notifications.js

@@ -5,7 +5,7 @@ import {
 	CmuxClient,
 	emitOsc777Notification,
 	resolveCmuxConfig,
-} from "../cmux/index.js";
+} from "./cmux/index.js";
 import { loadEffectiveSFPreferences } from "./preferences.js";
 /**
  * Send a native desktop notification. Non-blocking, non-fatal.

src/resources/extensions/sf/subagent/index.js

@@ -21,7 +21,7 @@ import { Type } from "@sinclair/typebox";
 import { StringEnum } from "@singularity-forge/ai";
 import { getMarkdownTheme } from "@singularity-forge/coding-agent";
 import { Container, Markdown, Spacer, Text } from "@singularity-forge/tui";
-import { CmuxClient, shellEscape } from "../../cmux/index.js";
+import { CmuxClient, shellEscape } from "../cmux/index.js";
 import { formatTokenCount } from "../../shared/mod.js";
 import { getCurrentPhase } from "../../shared/sf-phase-state.js";
 import { delay } from "../atomic-write.js";