feat(#3149): add dedicated init.debug entry point for /gsd:debug

/gsd:debug was one of the last workflows with no cmdInit* of its own: its
Step 0 made three separate round-trips (state.load, resolve-model
gsd-debugger, config-get workflow.tdd_mode) to assemble one context. Because
no debug-scoped fact was computed at any entry point, ADR-1671 admission gate
(2) could never be satisfied for debug — an applicability atom naming such a
fact would evaluate FALSE forever and silently exclude its section.

Adds cmdInitDebug (init.debug), registers it in the init router and the
command-alias table, and collapses debug.md Step 0 to one call. Every field
resolves through the same primitive the call it replaces used: loadConfig for
commit_docs, withProjectRoot for response_language (#2402), planningPaths for
debug_dir, resolveModelInternal for debugger_model, and the existing
Boolean(workflow.tdd_mode) idiom for tdd_mode.

PlanningPaths gains a debug field so state.load and init.debug share ONE
debug-directory expression rather than two kept in sync by hand. state.load
keeps emitting debug_dir: it is a shipped query surface with its own test
anchor, so narrowing it would break unseen consumers for no gain.

No WHEN_VOCABULARY atom and no gsd:section marker: gate (1), a consuming
section of at least 400 bytes, belongs to the change that adds the section.

Closes #3149

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

src/planning-workspace.cts

@@ -166,6 +166,7 @@ interface PlanningPaths {
   config: string;
   phases: string;
   requirements: string;
+  debug: string;
 }
 
 function planningPaths(cwd: string, ws?: string | null): PlanningPaths {
@@ -178,6 +179,10 @@ function planningPaths(cwd: string, ws?: string | null): PlanningPaths {
     config: path.join(base, 'config.json'),
     phases: path.join(base, 'phases'),
     requirements: path.join(base, 'REQUIREMENTS.md'),
+    // #3149: the debug-session directory. Single source for both `state.load`'s
+    // `debug_dir` field and `init.debug`'s — previously each composed its own
+    // `path.join(planning, 'debug')` (DEFECT.GENERATIVE-FIX).
+    debug: path.join(base, 'debug'),
   };
 }