feat: skill prefix is now a persistent user choice (v0.12.11.0) (#571)

* feat: make skill prefix a persistent, interactive user setting

- Add --prefix flag alongside --no-prefix
- Read/write skill_prefix from ~/.gstack/config.yaml (true/false)
- Interactive prompt on first setup when no preference saved
- Non-TTY environments default to flat names (no prefix)
- Add cleanup_prefixed_claude_symlinks() for reverse direction
- Fix gstack-config sed portability (mktemp+mv instead of BSD sed -i '')
- Add SKILL_PREFIX to preamble output with namespace-aware instruction

* test: add prefix config tests + README switching instructions

8 structural tests for persistent prefix setting:
config reading, --prefix flag, config persistence, interactive
prompt, TTY fallback, reverse cleanup, cleanup ordering, welcome.

* chore: regenerate SKILL.md files with SKILL_PREFIX preamble

* chore: bump version and changelog (v0.12.11.0)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: reframe changelog as feature, not mea culpa

* docs: update CONTRIBUTING + CLAUDE.md for prefix-aware vendoring

- CONTRIBUTING: vendoring now includes ./setup step for per-skill symlinks
- CONTRIBUTING: prefix choice documented in contributor workflow + dev diagram
- CONTRIBUTING: switching prefix mode section added
- CLAUDE.md: vendored symlink awareness section covers prefix setting

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

bin/gstack-config

@@ -23,7 +23,9 @@ case "${1:-}" in
     VALUE="${3:?Usage: gstack-config set <key> <value>}"
     mkdir -p "$STATE_DIR"
     if grep -qE "^${KEY}:" "$CONFIG_FILE" 2>/dev/null; then
-      sed -i '' "s/^${KEY}:.*/${KEY}: ${VALUE}/" "$CONFIG_FILE"
+      # Portable in-place edit (BSD sed uses -i '', GNU sed uses -i without arg)
+      _tmpfile="$(mktemp "${CONFIG_FILE}.XXXXXX")"
+      sed "s/^${KEY}:.*/${KEY}: ${VALUE}/" "$CONFIG_FILE" > "$_tmpfile" && mv "$_tmpfile" "$CONFIG_FILE"
     else
       echo "${KEY}: ${VALUE}" >> "$CONFIG_FILE"
     fi