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>

.changeset/nimble-ravens-dart.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 2300
+---
+**`/gsd-debug` now auto-resumes instead of stopping mid-investigation** — when the debug session-manager's own turn ended before the investigation was complete, the orchestrator treated the intermediate progress summary as completion and returned control to the user. It now recognizes a non-terminal `CONTINUE_REQUIRED` return, auto-resumes from the on-disk checkpoint, and only stops for genuine terminal conditions (with a no-progress anti-loop guard). (#2257)