chore(#1544): test-quality + doc cleanups from the #1507 conversion-module review (#1547)

Follow-up hygiene from the #1507 epic review (release blocker fixed in #1537):

- path-replacement.test.cjs: delete the hand-reimplemented computePathPrefix copy
  (ADR-1508 said to); route all cases (incl. Windows + outside-home) through the
  real _computePathPrefix so the test can no longer drift from the function.
- enh-1511: add an isWindowsHost no-op tripwire characterization test.
- enh-1511: add a deterministic (fs-method monkeypatch, root/OS-independent)
  error-path test for applyRuntimeContentRewritesForCommandsInPlace — asserts the
  temp dir is rm'd on a read failure with no orphaned gsd-cmd-rewrites-* leak.
- runtime-artifact-conversion.cts: @internal note on rewriteStagedCommandBodies
  (deep-seam companion to rewriteStagedSkillBodies; no production caller today).
- ADR-1508 + CONTEXT.md: qualify the "single owner" claim with the deliberate
  opencode/kilo applyOpencodeFamilyPathPrefix pre-conversion carve-out (#784).

No user-facing behavior change (tests + internal docs + one code comment).
Refs #1507.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/runtime-artifact-conversion.cts

@@ -2444,6 +2444,11 @@ function rewriteStagedSkillBodies(stagedDir, opts) {
  * attribution from opts, then delegates to applyRuntimeContentRewritesForCommandsInPlace
  * (single copy+rewrite owner).
  *
+ * @internal — symmetric companion to rewriteStagedSkillBodies; retained as the deep-seam
+ * API for command bodies. No production caller today (install rewrites commands via
+ * copyWithPathReplacement → applyRuntimeContentRewritesForCommandsInPlace). Kept for
+ * API symmetry + test coverage.
+ *
  * @returns {string} path to the temp dir (caller is responsible for cleanup)
  */
 function rewriteStagedCommandBodies(stagedDir, opts) {