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>

tests/workflow-size-baseline.json

@@ -8,14 +8,14 @@
   "audit-fix.md": 10988,
   "audit-milestone.md": 17637,
   "audit-uat.md": 7425,
-  "autonomous.md": 42275,
+  "autonomous.md": 42778,
   "check-todos.md": 9431,
   "cleanup.md": 9897,
   "code-review-fix.md": 23890,
   "code-review.md": 31602,
   "complete-milestone.md": 29987,
   "debug.md": 13505,
-  "diagnose-issues.md": 12762,
+  "diagnose-issues.md": 12820,
   "discovery-phase.md": 8651,
   "discuss-phase-assumptions.md": 26984,
   "discuss-phase-power.md": 11273,
@@ -24,7 +24,7 @@
   "docs-update.md": 55662,
   "edit-phase.md": 12883,
   "eval-review.md": 9923,
-  "execute-phase.md": 92926,
+  "execute-phase.md": 93024,
   "execute-plan.md": 31365,
   "explore.md": 10497,
   "extract-learnings.md": 12849,
@@ -39,7 +39,7 @@
   "insert-phase.md": 8943,
   "list-phase-assumptions.md": 4305,
   "list-workspaces.md": 5655,
-  "manager.md": 25949,
+  "manager.md": 26265,
   "map-codebase.md": 20789,
   "milestone-summary.md": 11774,
   "mvp-phase.md": 13582,
@@ -57,7 +57,7 @@
   "pr-branch.md": 9561,
   "profile-user.md": 20650,
   "progress.md": 29387,
-  "quick.md": 48772,
+  "quick.md": 48830,
   "reapply-patches.md": 20393,
   "remove-phase.md": 8469,
   "remove-workspace.md": 7507,