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

scripts/resolvers/preamble.ts

@@ -24,8 +24,10 @@ _PROACTIVE=$(${ctx.paths.binDir}/gstack-config get proactive 2>/dev/null || echo
 _PROACTIVE_PROMPTED=$([ -f ~/.gstack/.proactive-prompted ] && echo "yes" || echo "no")
 _BRANCH=$(git branch --show-current 2>/dev/null || echo "unknown")
 echo "BRANCH: $_BRANCH"
+_SKILL_PREFIX=$(${ctx.paths.binDir}/gstack-config get skill_prefix 2>/dev/null || echo "false")
 echo "PROACTIVE: $_PROACTIVE"
 echo "PROACTIVE_PROMPTED: $_PROACTIVE_PROMPTED"
+echo "SKILL_PREFIX: $_SKILL_PREFIX"
 source <(${ctx.paths.binDir}/gstack-repo-mode 2>/dev/null) || true
 REPO_MODE=\${REPO_MODE:-unknown}
 echo "REPO_MODE: $REPO_MODE"
@@ -51,6 +53,11 @@ types (e.g., /qa, /ship). If you would have auto-invoked a skill, instead briefl
 "I think /skillname might help here — want me to run it?" and wait for confirmation.
 The user opted out of proactive behavior.
 
+If \`SKILL_PREFIX\` is \`"true"\`, the user has namespaced skill names. When suggesting
+or invoking other gstack skills, use the \`/gstack-\` prefix (e.g., \`/gstack-qa\` instead
+of \`/qa\`, \`/gstack-ship\` instead of \`/ship\`). Disk paths are unaffected — always use
+\`${ctx.paths.skillRoot}/[skill-name]/SKILL.md\` for reading skill files.
+
 If output shows \`UPGRADE_AVAILABLE <old> <new>\`: read \`${ctx.paths.skillRoot}/gstack-upgrade/SKILL.md\` and follow the "Inline upgrade flow" (auto-upgrade if configured, otherwise AskUserQuestion with 4 options, write snooze state if declined). If \`JUST_UPGRADED <from> <to>\`: tell user "Running gstack v{to} (just updated!)" and continue.`;
 }