fix(render): section refs point at the FINAL render dir, never the tmp swap dir (#2692)

gen-skill-docs bakes its --out-dir into rendered CONTENT (rewriteSectionBase),
and both swap-in callers (setup, gstack-config gbrain-refresh) render into
claude.tmp.<pid> before the #2569 atomic rename — so every rendered skill
carried ~9 dead section Read paths that pointed at a directory the swap had
just deleted. New --link-root flag names the final serving dir (defaults to
--out-dir for direct-render callers: bin/dev-setup, dev-skill.ts, mkdtemp
tests — full caller audit in the wave notes); the rewrite now uses a
replacement callback so a $-bearing configured path can't expand as $& in a
replacement string. The swap logic itself stays byte-identical. Tests pin the
generator contract (tmp out-dir files reference the final dir, $-bearing
path included) and both callers' wiring.

Fixes #2692

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

setup

@@ -2162,7 +2162,7 @@ if [ -x "$DETECT_BIN" ]; then
           # No pipe before the || guard: `cmd | tail -3` reports TAIL's exit
           # status, so a generator crash read as success (same masking the
           # main gen:skill-docs site had). Capture, show the tail, propagate.
-          _GEN_USER_OUT=$(bun_cmd run gen:skill-docs:user --host claude --out-dir "$_GSTACK_RENDER_TMP" 2>&1)
+          _GEN_USER_OUT=$(bun_cmd run gen:skill-docs:user --host claude --out-dir "$_GSTACK_RENDER_TMP" --link-root "$_GSTACK_RENDER_DIR" 2>&1)
           _GEN_USER_RC=$?
           printf '%s\n' "$_GEN_USER_OUT" | tail -3
           exit "$_GEN_USER_RC"