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>

docs/adr/1508-runtime-artifact-conversion-module.md

@@ -14,7 +14,7 @@ This is the **last upward dependency from the `.cts` source tree into the hand-a
 
 ## Decision
 
-- Promote the `[Planned]` **Runtime Artifact Conversion Module** (`src/runtime-artifact-conversion.cts`) to the single owner of per-runtime **content rewriting**: the per-runtime converters (already relocated as ADR-3660's "first slice", #1099), **plus** the rewrite engine `_applyRuntimeRewrites`, the staged-content walkers, path-prefix derivation, and commit attribution. The **Runtime Artifact Layout Module** keeps owning **placement** only.
+- Promote the `[Planned]` **Runtime Artifact Conversion Module** (`src/runtime-artifact-conversion.cts`) to the single owner of per-runtime **content rewriting**: the per-runtime converters (already relocated as ADR-3660's "first slice", #1099), **plus** the rewrite engine `_applyRuntimeRewrites`, the staged-content walkers, path-prefix derivation, and commit attribution. The **Runtime Artifact Layout Module** keeps owning **placement** only. **Exception:** opencode and kilo path-prefix rewriting remains a deliberate `bin/install.js`-owned pre-conversion step (see `applyOpencodeFamilyPathPrefix`); this is intentional per #784 and is not a violation of the single-owner rule.
 - **Public seam** — two deep calls; the caller passes only what it has, the module derives the rest:
   - `rewriteStagedSkillBodies(stagedDir, { runtime, configDir, scope }, env?)` — in-place walk (skills / kimi-agents).
   - `rewriteStagedCommandBodies(stagedDir, { runtime, configDir, scope }, env?) → tempDir` — copy-to-temp (commands).