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>

docs/how-to/work-in-parallel-with-workstreams.md

@@ -60,6 +60,8 @@ Shows all workstreams and which one is currently active in your session.
 
 From this point forward, all GSD workflow commands operate in the `backend-api` context. The switch is session-scoped: when multiple Claude Code terminals are open on the same repo, each session can hold a different active workstream without interfering with the others.
 
+The statusline's GSD-state segment (milestone, phase, progress) reflects whichever workstream resolves as active — the same `GSD_WORKSTREAM` env var / stored pointer precedence every workstream-aware command uses. If no workstream can be resolved in a workstream-mode project, the segment shows `no active workstream` rather than disappearing silently.
+
 Once switched, drive the normal phase workflow:
 
 ```bash