feat(coding-agent): add FooterDataProvider for git branch and extension statuses

Expose data that extensions cannot otherwise access: git branch and
extension statuses from setStatus(). Token stats, model info, etc.
remain computable via ctx.sessionManager and ctx.model.

packages/coding-agent/src/core/extensions/types.ts

@@ -22,6 +22,7 @@ import type { BashResult } from "../bash-executor.js";
 import type { CompactionPreparation, CompactionResult } from "../compaction/index.js";
 import type { EventBus } from "../event-bus.js";
 import type { ExecOptions, ExecResult } from "../exec.js";
+import type { ReadonlyFooterDataProvider } from "../footer-data-provider.js";
 import type { KeybindingsManager } from "../keybindings.js";
 import type { CustomMessage } from "../messages.js";
 import type { ModelRegistry } from "../model-registry.js";
@@ -82,8 +83,17 @@ export interface ExtensionUIContext {
 	setWidget(key: string, content: string[] | undefined): void;
 	setWidget(key: string, content: ((tui: TUI, theme: Theme) => Component & { dispose?(): void }) | undefined): void;
 
-	/** Set a custom footer component, or undefined to restore the built-in footer. */
-	setFooter(factory: ((tui: TUI, theme: Theme) => Component & { dispose?(): void }) | undefined): void;
+	/** Set a custom footer component, or undefined to restore the built-in footer.
+	 *
+	 * The factory receives a FooterDataProvider for data not otherwise accessible:
+	 * git branch and extension statuses from setStatus(). Token stats, model info,
+	 * etc. are available via ctx.sessionManager and ctx.model.
+	 */
+	setFooter(
+		factory:
+			| ((tui: TUI, theme: Theme, footerData: ReadonlyFooterDataProvider) => Component & { dispose?(): void })
+			| undefined,
+	): void;
 
 	/** Set a custom header component (shown at startup, above chat), or undefined to restore the built-in header. */
 	setHeader(factory: ((tui: TUI, theme: Theme) => Component & { dispose?(): void }) | undefined): void;

packages/coding-agent/src/core/footer-data-provider.ts

@@ -0,0 +1,121 @@
+import { existsSync, type FSWatcher, readFileSync, statSync, watch } from "fs";
+import { dirname, join, resolve } from "path";
+
+/**
+ * Find the git HEAD path by walking up from cwd.
+ * Handles both regular git repos (.git is a directory) and worktrees (.git is a file).
+ */
+function findGitHeadPath(): string | null {
+	let dir = process.cwd();
+	while (true) {
+		const gitPath = join(dir, ".git");
+		if (existsSync(gitPath)) {
+			try {
+				const stat = statSync(gitPath);
+				if (stat.isFile()) {
+					const content = readFileSync(gitPath, "utf8").trim();
+					if (content.startsWith("gitdir: ")) {
+						const gitDir = content.slice(8);
+						const headPath = resolve(dir, gitDir, "HEAD");
+						if (existsSync(headPath)) return headPath;
+					}
+				} else if (stat.isDirectory()) {
+					const headPath = join(gitPath, "HEAD");
+					if (existsSync(headPath)) return headPath;
+				}
+			} catch {
+				return null;
+			}
+		}
+		const parent = dirname(dir);
+		if (parent === dir) return null;
+		dir = parent;
+	}
+}
+
+/**
+ * Provides git branch and extension statuses - data not otherwise accessible to extensions.
+ * Token stats, model info available via ctx.sessionManager and ctx.model.
+ */
+export class FooterDataProvider {
+	private extensionStatuses = new Map<string, string>();
+	private cachedBranch: string | null | undefined = undefined;
+	private gitWatcher: FSWatcher | null = null;
+	private branchChangeCallbacks = new Set<() => void>();
+
+	constructor() {
+		this.setupGitWatcher();
+	}
+
+	/** Current git branch, null if not in repo, "detached" if detached HEAD */
+	getGitBranch(): string | null {
+		if (this.cachedBranch !== undefined) return this.cachedBranch;
+
+		try {
+			const gitHeadPath = findGitHeadPath();
+			if (!gitHeadPath) {
+				this.cachedBranch = null;
+				return null;
+			}
+			const content = readFileSync(gitHeadPath, "utf8").trim();
+			this.cachedBranch = content.startsWith("ref: refs/heads/") ? content.slice(16) : "detached";
+		} catch {
+			this.cachedBranch = null;
+		}
+		return this.cachedBranch;
+	}
+
+	/** Extension status texts set via ctx.ui.setStatus() */
+	getExtensionStatuses(): ReadonlyMap<string, string> {
+		return this.extensionStatuses;
+	}
+
+	/** Subscribe to git branch changes. Returns unsubscribe function. */
+	onBranchChange(callback: () => void): () => void {
+		this.branchChangeCallbacks.add(callback);
+		return () => this.branchChangeCallbacks.delete(callback);
+	}
+
+	/** Internal: set extension status */
+	setExtensionStatus(key: string, text: string | undefined): void {
+		if (text === undefined) {
+			this.extensionStatuses.delete(key);
+		} else {
+			this.extensionStatuses.set(key, text);
+		}
+	}
+
+	/** Internal: cleanup */
+	dispose(): void {
+		if (this.gitWatcher) {
+			this.gitWatcher.close();
+			this.gitWatcher = null;
+		}
+		this.branchChangeCallbacks.clear();
+	}
+
+	private setupGitWatcher(): void {
+		if (this.gitWatcher) {
+			this.gitWatcher.close();
+			this.gitWatcher = null;
+		}
+
+		const gitHeadPath = findGitHeadPath();
+		if (!gitHeadPath) return;
+
+		try {
+			this.gitWatcher = watch(gitHeadPath, () => {
+				this.cachedBranch = undefined;
+				for (const cb of this.branchChangeCallbacks) cb();
+			});
+		} catch {
+			// Silently fail if we can't watch
+		}
+	}
+}
+
+/** Read-only view for extensions - excludes setExtensionStatus and dispose */
+export type ReadonlyFooterDataProvider = Pick<
+	FooterDataProvider,
+	"getGitBranch" | "getExtensionStatuses" | "onBranchChange"
+>;