fix(#1521): resolve own runtime + worktrees-off for all non-Claude installs (#1537)

* fix(#1521): resolve own runtime + worktrees-off for all non-Claude installs

Generalizes the Codex-only #1515/#1519 fix to every non-Claude runtime, and
wires it into the real install path (where it was previously dead-on-arrival).

Root causes:
1. The runtime-default stamping lived only in `_applyRuntimeRewrites`, but the
   installer emits `gsd-core/workflows/*.md` via `copyWithPathReplacement`, which
   never calls it — so a real `--codex`/`--cursor`/etc. install emitted
   `--default claude` and worktrees-on. RUNTIME mis-resolved to claude and the
   workflow ran executors unisolated against the main checkout. (#1515/#1519 were
   also dead-on-arrival in real installs; this repairs them.)
2. Only `case 'codex'` was stamped; every other non-Claude runtime kept the
   Claude default.

Fix:
- New `_stampNonClaudeRuntimeDefaults(content, runtime)` (single shared helper)
  stamps `--default <runtime>` + `use_worktrees=false` for every `runtime !=
  claude`; called from both `_applyRuntimeRewrites` and, crucially,
  `copyWithPathReplacement` in bin/install.js (the real workflow emit path).
- Generalize the fail-closed worktree guard `= codex` -> `!= claude` in
  execute-phase/quick/diagnose-issues (worktree isolation is Claude-Code-only).
- Flip manager/autonomous inline-vs-background gating to `codex -> background,
  everything-else -> inline` (research: only Codex can background-nest the
  pipeline's subagents; all others run inline, which they support).

Worktree-capability determination is research-backed (official docs for all 14
non-Claude runtimes: none honor GSD's isolation="worktree" mechanism, only Codex
background-nests). New end-to-end real-install test asserts the EMITTED workflow
is stamped — the regression guard that would have caught the dead-on-arrival bug.

Closes #1521

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013vX5eUtWa2wsZEyeMf5i3r

* chore(#1521): backfill changeset PR number (#1537)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013vX5eUtWa2wsZEyeMf5i3r

---------

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

bin/install.js

@@ -6698,6 +6698,7 @@ function migrateLegacyDevPreferencesToSkill(targetDir, saved, runtime, scope = '
 // reference-identical to the conversion module (consistent with the walkers above).
 // All call sites are below this line → no TDZ hazard.
 const _applyRuntimeRewrites = runtimeArtifactConversion._applyRuntimeRewrites;
+const _stampNonClaudeRuntimeDefaults = runtimeArtifactConversion._stampNonClaudeRuntimeDefaults;
 
 /**
  * Copy a staged directory's contents into destDir.
@@ -7289,6 +7290,15 @@ function copyWithPathReplacement(srcDir, destDir, pathPrefix, runtime, isCommand
       }
       content = processAttribution(content, getCommitAttribution(runtime));
 
+      // #1521: stamp the workflow runtime-resolution block so every non-Claude
+      // install resolves its own runtime identity and defaults use_worktrees=false.
+      // copyWithPathReplacement is the emit path for gsd-core/workflows/*.md;
+      // _applyRuntimeRewrites is NOT invoked here, so this is what makes the fix
+      // live in real installs (it is a no-op for files without those lines).
+      if (runtime !== 'claude') {
+        content = _stampNonClaudeRuntimeDefaults(content, runtime);
+      }
+
       // #3683 — normalize /gsd:<cmd> → /gsd-<cmd> in any body passing through
       // copyWithPathReplacement for runtimes that register commands under the
       // hyphen form; normalizeAgentBodyForRuntime self-gates on