fix(sdk): make state.complete-phase idempotent (#3489)

Re-invoking `state complete-phase --phase <N>` on a phase that was
already marked complete in STATE.md silently rolled STATE.md back to
that phase's moment-of-completion — clobbering Status, Last Activity,
Last Activity Description, and the ## Current Position body. The bug
fired whenever a follow-up phase had been inserted (or the next phase
had begun) and a downstream workflow re-ran complete-phase on the
already-closed phase. Damage was silent: handler reported
{"updated":["Status","Last Activity","Current Position"]} and no error.

Root cause: cmdStateCompletePhase wrote unconditionally — it never
consulted STATE.md to detect that the requested phase had been
superseded. The handler is a legacy-bridge fallback (no native SDK
registration), so the SDK CLI fell through to gsd-tools.cjs.

Fix: add an idempotency guard at the top of cmdStateCompletePhase.
If STATE.md's canonical Current Phase field already names a phase
distinct from the one we are being asked to mark complete, return a
no-op payload ({updated:[], phase:"<N>", idempotent:true, note:"phase
already superseded; no-op"}) without writing to STATE.md.

The guard is conservative — it only fires when Current Phase is set
and differs from the resolved target. First-time completion (Current
Phase == target, or Current Phase absent) is unaffected, so the four
existing complete-phase test cases (#2761, #3063) continue to pass.

Regression test: tests/bug-3489-complete-phase-idempotent.test.cjs
- re-running complete-phase --phase 02.2 with Current Phase=02.2.1
  in STATE.md leaves the file byte-identical and reports idempotent:true
- normal first-time completion is NOT flagged idempotent

Scope: handler-level idempotency only. Does not address the related
stopped_at filename-sort ordering issue called out in the bug report
(filed under suggested fix #2) or porting to the native registry.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

get-shit-done/bin/lib/state.cjs

@@ -1805,6 +1805,27 @@ function cmdStateCompletePhase(cwd, raw, overridePhase) {
     return;
   }
 
+  // Idempotency guard (#3489). If STATE.md's canonical `Current Phase` field
+  // already names a phase distinct from the one we are being asked to mark
+  // complete, the project has advanced past the requested phase (e.g. a
+  // follow-up phase was inserted, or the next phase began). Re-running
+  // `state complete-phase --phase <N>` in that situation previously rolled
+  // STATE.md back to <N>'s moment-of-completion — silently clobbering Status,
+  // Last Activity, Last Activity Description, and the Current Position body.
+  // The handler is now a no-op in that case so re-invocation from downstream
+  // workflows cannot regress the project state.
+  const existingCurrentPhaseRaw = stateExtractField(content, 'Current Phase') || '';
+  const existingCurrentPhaseMatch = String(existingCurrentPhaseRaw).match(/(\d+[A-Z]?(?:\.\d+)*)/i);
+  const existingCurrentPhase = existingCurrentPhaseMatch ? existingCurrentPhaseMatch[1] : null;
+  if (existingCurrentPhase && existingCurrentPhase !== resolvedPhase) {
+    output(
+      { updated: [], phase: resolvedPhase, idempotent: true, note: 'phase already superseded; no-op' },
+      raw,
+      'false',
+    );
+    return;
+  }
+
   const today = new Date().toISOString().split('T')[0];
   const updated = [];