fix(#2850): resolve the active workstream in the statusline GSD-state segment (#3012)

* test(#2850): add failing-first tests for workstream statusline state

readGsdState only ever reads the flat .planning/STATE.md via a directory
walk-up; it has no path for .planning/workstreams/<ws>/STATE.md and never
consults GSD_WORKSTREAM or the stored active-workstream pointer, so the
GSD-state segment silently disappears in workstream mode. These tests
prove the RED before the fix lands.

Uses shared saveSessionEnv/restoreSessionEnv/clearSessionEnv helpers now
added to tests/helpers.cjs (single source of truth for the session-env-var
save/clear/restore pattern also used by tests/active-workstream-store.unit.test.cjs,
which is updated here to consume the same shared helpers instead of its own
local, already-diverged copy).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* fix(#2850): resolve active workstream in the statusline

readGsdState only ever walked up looking for a flat .planning/STATE.md; it
had no branch for .planning/workstreams/<ws>/STATE.md and never consulted
GSD_WORKSTREAM or the stored active-workstream pointer, so the GSD-state
segment silently vanished in workstream-mode projects with no root
STATE.md (exit 0, no diagnostic).

Reuses the existing CLI>env>store resolution seam (resolveActiveWorkstream,
active-workstream-store.cts) and the existing mode-detection/path-building
seam (listAvailableWorkstreams/planningPaths, planning-workspace.cts)
rather than re-implementing either inline. When workstream mode is
detected but nothing resolves, readGsdState now returns a
{noActiveWorkstream:true} sentinel that formatGsdState/formatGsdStateCompact
render as "no active workstream" -- observable, never silent emptiness.
Flat-mode behavior and the case where a resolved workstream has no
STATE.md yet are both unchanged.

Adds active-workstream-store.cts's peekActiveWorkstream: a read-only
sibling of getActiveWorkstream. resolveActiveWorkstream's default store
lookup self-heals a stale/invalid pointer by deleting it
(adapter.clear()) -- correct for a command, but not for a renderer
invoked once per prompt, which must never mutate persistent, possibly
cross-session state as a side effect of drawing a screen. The statusline
now injects peekActiveWorkstream via resolveActiveWorkstream's own
getStored override, keeping the env>store precedence itself fully reused
while removing only the store tier's write side effect. This satisfies
the issue's AC4 ("the fix is purely additive to what's displayed").

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* chore(#2850): backfill changeset PR number to 3012

---------

Co-authored-by: sim <sim@local>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

src/active-workstream-store.cts

@@ -232,6 +232,31 @@ function getActiveWorkstream(cwd: string, opts: ActiveWorkstreamOpts = {}): stri
   return name;
 }
 
+/**
+ * Read-only sibling of getActiveWorkstream (#2850): identical resolution —
+ * adapter -> stored name -> validate format -> workstream dir exists — but
+ * NEVER calls adapter.clear(). getActiveWorkstream's self-heal (deleting a
+ * stale/invalid pointer) is correct for a command that is actively acting on
+ * the active workstream; it is wrong for a read-only consumer invoked on
+ * every render (e.g. the statusline hook), which must never mutate
+ * persistent, possibly cross-session state as a side effect of drawing a
+ * screen. A stale or invalid pointer simply resolves to null here — the
+ * caller decides what "unresolvable" means for its own render, and the
+ * pointer file is left exactly as it was for whatever created it to fix.
+ */
+function peekActiveWorkstream(cwd: string, opts: ActiveWorkstreamOpts = {}): string | null {
+  const adapter = pickActiveWorkstreamAdapter(cwd, opts);
+  if (!adapter) return null;
+
+  const name = adapter.read();
+  if (!name || !validateWorkstreamName(name)) return null;
+
+  const wsDir = path.join(planningRoot(cwd), 'workstreams', name);
+  if (!fs.existsSync(wsDir)) return null;
+
+  return name;
+}
+
 function setActiveWorkstream(cwd: string, name: string | null | undefined, opts: ActiveWorkstreamOpts = {}): void {
   const adapter = pickActiveWorkstreamAdapter(cwd, opts);
   if (!adapter) return;
@@ -348,6 +373,7 @@ export = {
   createMemoryPointerAdapter,
   pickActiveWorkstreamAdapter,
   getActiveWorkstream,
+  peekActiveWorkstream,
   setActiveWorkstream,
   clearActiveWorkstream,
   parseCliWorkstream,