fix(#2257): auto-resume debug on non-terminal session-manager return (#2300)

The /gsd-debug orchestrator handled the gsd-debug-session-manager return
with only two literal-string checks (DEBUG SESSION COMPLETE, ABANDONED)
and no else branch, so a usable-but-non-terminal progress summary (the
manager's own turn/context budget exhausted mid-loop, with a valid
on-disk checkpoint) fell through to the user as if the debug were
complete. Same gap at the continue subcommand.

Callee side (agents/gsd-debug-session-manager.md): add an explicit
non-terminal CONTINUE_REQUIRED return marker, distinct from the two
terminal shapes and from a genuine user-input checkpoint.

Orchestrator (gsd-core/workflows/debug.md Sections 4 and 1c): classify
returns exhaustively — recognized terminal markers behave as before,
anything else is non-terminal and auto-resumes by re-spawning the
session manager from the same slug/checkpoint. Anti-loop guard: after
two consecutive no-progress resumes (unchanged next_action/updated),
emit a blocker report instead of looping.

Regression test (source-text contract guard, fix-2196 idiom) asserts
both sections' non-terminal/auto-resume branch, the CONTINUE_REQUIRED
marker, and the anti-loop bound.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/workflow-size-baseline.json

@@ -14,7 +14,7 @@
   "code-review-fix.md": 24320,
   "code-review.md": 31916,
   "complete-milestone.md": 31071,
-  "debug.md": 14241,
+  "debug.md": 19031,
   "diagnose-issues.md": 12864,
   "discovery-phase.md": 8651,
   "discuss-phase-assumptions.md": 27302,