feat: gpt-5.6-sol bounded-scope overlay + scope-aware resolvers

The Sol profile pins the explicit task as the lake: adjacent work is
report-only, investigation is bounded, runs terminate on one clean
verification pass, and the AskUserQuestion decision-brief format is never
trimmed. The overlay wrapper grants scope-interpretation precedence while
concrete workflow steps, gates, and skill-mandated re-verification loops
still win. Sol-specific Completeness Principle and first-run intro copy.
New SETUP_COMMAND resolver renders './setup --host <host>' for every
non-claude host so generated upgrade skills reinstall their own host.

gstack-upgrade/SKILL.md.tmpl

@@ -138,7 +138,7 @@ git checkout -- 'SKILL.md' '*/SKILL.md' '*/sections/*.md' 2>/dev/null || true
 STASH_OUTPUT=$(git stash 2>&1)
 git fetch origin
 git reset --hard origin/main
-./setup
+{{SETUP_COMMAND}}
 ```
 If `$STASH_OUTPUT` contains "Saved working directory", warn the user: "Note: local changes were stashed (any modified generated SKILL.md/sections files were discarded first — they regenerate on setup). Run `git stash pop` in the skill directory to restore your own changes."
 
@@ -149,7 +149,7 @@ TMP_DIR=$(mktemp -d)
 git clone --depth 1 https://github.com/garrytan/gstack.git "$TMP_DIR/gstack"
 mv "$INSTALL_DIR" "$INSTALL_DIR.bak"
 mv "$TMP_DIR/gstack" "$INSTALL_DIR"
-cd "$INSTALL_DIR" && ./setup
+cd "$INSTALL_DIR" && {{SETUP_COMMAND}}
 rm -rf "$INSTALL_DIR.bak" "$TMP_DIR"
 ```
 
@@ -189,7 +189,7 @@ Tell user: "Removed vendored copy at `$LOCAL_GSTACK` (team mode active — globa
 mv "$LOCAL_GSTACK" "$LOCAL_GSTACK.bak"
 cp -Rf "$INSTALL_DIR" "$LOCAL_GSTACK"
 rm -rf "$LOCAL_GSTACK/.git"
-cd "$LOCAL_GSTACK" && ./setup
+cd "$LOCAL_GSTACK" && {{SETUP_COMMAND}}
 rm -rf "$LOCAL_GSTACK.bak"
 ```
 Tell user: "Also updated vendored copy at `$LOCAL_GSTACK` — commit `.claude/skills/gstack/` when you're ready."