fix(#1098): guard generate-claude-md against clobbering hand-crafted files; redirect default to .claude/CLAUDE.md (#1118)

/gsd-new-project wrote a repo-root CLAUDE.md full of broad project docs,
overwriting/diluting a hand-crafted instruction file. --force was parsed but
silently dropped, and nothing guarded an existing non-GSD file.

- Guard: an existing instruction file with no `<!-- GSD:<section>-start` markers
  (hand-crafted) is left untouched; report action:"skipped". --force (now wired
  through CmdGenerateClaudeMdOptions) overwrites intentionally. The marker check
  uses /<!-- GSD:[a-z]+-start/ so a file merely documenting GSD syntax is safe.
- Redirect: the Claude-family default output is now ./.claude/CLAUDE.md (a valid
  auto-loaded project-memory location) instead of repo-root ./CLAUDE.md. Aligned
  across the handler default, config-defaults.manifest.json, buildNewProjectConfig,
  the config template, new-project.md, and cmdGenerateClaudeProfile; advisory
  read-CLAUDE.md hints in plan-phase/quick/profile-user updated. Codex still
  writes AGENTS.md.

Closes #1098

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/workflow-size-baseline.json

@@ -44,20 +44,20 @@
   "milestone-summary.md": 11774,
   "mvp-phase.md": 13582,
   "new-milestone.md": 32422,
-  "new-project.md": 61690,
+  "new-project.md": 61802,
   "new-workspace.md": 11254,
   "next.md": 17868,
   "node-repair.md": 4173,
   "note.md": 6563,
   "pause-work.md": 13654,
   "plan-milestone-gaps.md": 11765,
-  "plan-phase.md": 94253,
+  "plan-phase.md": 94343,
   "plan-review-convergence.md": 22949,
   "plant-seed.md": 11741,
   "pr-branch.md": 4994,
-  "profile-user.md": 20457,
+  "profile-user.md": 20650,
   "progress.md": 29387,
-  "quick.md": 46213,
+  "quick.md": 46282,
   "reapply-patches.md": 20393,
   "remove-phase.md": 8469,
   "remove-workspace.md": 7507,