docs: relocate v0 design specs to docs/design/

I moved both v0 design specs out of working/ and into docs/design/. That's the conventional permanent home for project documentation, where engineers will look during implementation. working/ is meant for transient in-progress artifacts that file away once the work ships, and these specs are long-lived design docs that don't fit that contract.

Files moved:
- working/stats-dashboard/stats-dashboard.org → docs/design/stats-dashboard.org
- working/fsrs-spec/fsrs-spec.org → docs/design/fsrs-spec.org

The git rename detection picked both up, so file history follows the move.

I also dropped the stale /docs entry from .gitignore. The Makefile doesn't write to docs/ and nothing else references it as a build output, so the ignore was inherited cruft that would have silently dropped any tracked file under docs/.

I updated path references in seven spots: three docstring/comment refs in org-drill.el, one in tests/test-org-drill-session-record.el (the Commentary block), and three inside the specs themselves. Two refs in fsrs-spec.org now point at the correct location for its defcustom docstring and option description. One in stats-dashboard.org's References section points at the sister spec.

Full make test-unit green. eask compile clean.

1 files changed, 3 insertions, 4 deletions

diff --git a/org-drill.el b/org-drill.el
index d8f32ac..2680113 100644
--- a/org-drill.el
+++ b/org-drill.el
@@ -88,8 +88,7 @@
   "Persistent session log and the statistics dashboard.
 The dashboard is opt-in via \\='M-x org-drill-statistics\\='; the session log
 itself is updated automatically at the end of every completed (non-suspended)
-drill session.  See working/stats-dashboard/stats-dashboard.org for the
-design rationale."
+drill session.  See docs/design/stats-dashboard.org for the design rationale."
   :tag "Org-Drill Statistics"
   :group 'org-drill)
 
@@ -567,7 +566,7 @@ pace of learning.")
      "Persistent matrix of optimal factors (fallback after load failure).")))
 
 ;; Statistics session log.  Records one entry per completed drill session
-;; so the stats dashboard (working/stats-dashboard/stats-dashboard.org) has
+;; so the stats dashboard (docs/design/stats-dashboard.org) has
 ;; a temporal axis to render trends against.  Same persist-defvar +
 ;; condition-case pattern as the SM5 matrix above.  See
 ;; `org-drill--session-log-quarantine' for the corrupt-file recovery
@@ -576,7 +575,7 @@ pace of learning.")
 (cl-defstruct org-drill-session-record
   "One completed drill session, persisted to `org-drill-session-log'.
 Slots match the v0 stats-dashboard spec; see
-working/stats-dashboard/stats-dashboard.org for field semantics."
+docs/design/stats-dashboard.org for field semantics."
   start-time      ; float — `float-time' at session start
   end-time        ; float — `float-time' at session end
   scope           ; symbol or list — `org-drill-scope' at start