fix(coding-agent/session): suppressed subagent breadcrumbs so --continue resumes the parent session

- Added a `suppressBreadcrumb` option to `SessionManager.open()` so headless opens skip writing the per-TTY `--continue` breadcrumb, and passed it from the subagent opens in `task/executor.ts` and the HTML export open in `export/html/index.ts`, which run in the parent's terminal and were clobbering the breadcrumb with their own artifact-dir session file.
- Added `resolveBreadcrumbToInteractiveRoot()` and applied it in `continueRecent()` so already-poisoned breadcrumbs pointing inside a parent's artifacts dir (`<parent>/<agentId>.jsonl`) resolve back up to the top-level interactive session.
- Added `subagent-breadcrumb.test.ts` covering both that a subagent open keeps `--continue` on the parent and that a stale subagent-pointing breadcrumb is recovered.

packages/coding-agent/src/task/executor.ts

@@ -1927,7 +1927,12 @@ export async function runSubprocess(options: ExecutorOptions): Promise<SingleRes
 
 			const effectiveCwd = worktree ?? cwd;
 			const sessionManager = sessionFile
-				? await awaitAbortable(SessionManager.open(sessionFile, undefined, undefined, { initialCwd: effectiveCwd }))
+				? await awaitAbortable(
+						SessionManager.open(sessionFile, undefined, undefined, {
+							initialCwd: effectiveCwd,
+							suppressBreadcrumb: true,
+						}),
+					)
 				: SessionManager.inMemory(effectiveCwd);
 			if (options.parentArtifactManager) {
 				sessionManager.adoptArtifactManager(options.parentArtifactManager);
@@ -2048,7 +2053,9 @@ export async function runSubprocess(options: ExecutorOptions): Promise<SingleRes
 				// (createAgentSession → agent.replaceMessages). Isolated runs are not
 				// resumable (worktree is merged + cleaned) and never get a reviver.
 				reviveSession = async () => {
-					const reopened = await SessionManager.open(sessionFile);
+					const reopened = await SessionManager.open(sessionFile, undefined, undefined, {
+						suppressBreadcrumb: true,
+					});
 					if (options.parentArtifactManager) {
 						reopened.adoptArtifactManager(options.parentArtifactManager);
 					}