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/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"
+>;