fix(install): make Claude Code install/update idempotent across channels

install.sh aborted with npm EEXIST when claude was already present:
the binary is a native-installer symlink (~/.local/bin/claude ->
~/.local/share/claude/versions/*) that npm does not own, and the
npm prefix (~/.local, set for BLK-013) targets the same path. The
`else err` branch turned EEXIST into a fatal exit. No presence guard
existed, unlike the RTK/GSD steps.

- install.sh: skip-if-present guard (command -v claude), mirroring
  the RTK/GSD pattern; npm only runs on a truly fresh machine.
- update-all.sh: pick updater by channel — npm for npm-managed
  installs, `claude update` for native installs (npm would EEXIST).

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

update-all.sh

@@ -27,7 +27,15 @@ echo "── Updating Claude Code CLI..."
 if command -v claude &>/dev/null; then
   CURRENT_VER=$(claude --version 2>/dev/null | head -1 || echo "unknown")
   info "Current: $CURRENT_VER"
-  if npm install -g @anthropic-ai/claude-code@latest 2>/dev/null; then
+  # Use the updater that matches the install channel: npm-managed installs
+  # update via npm; native-installer installs self-update via `claude update`
+  # (npm would EEXIST on the ~/.local/bin/claude symlink it does not own).
+  if npm ls -g @anthropic-ai/claude-code &>/dev/null; then
+    UPDATE_CMD=(npm install -g @anthropic-ai/claude-code@latest)
+  else
+    UPDATE_CMD=(claude update)
+  fi
+  if "${UPDATE_CMD[@]}" &>/dev/null; then
     NEW_VER=$(claude --version 2>/dev/null | head -1 || echo "unknown")
     if [ "$CURRENT_VER" = "$NEW_VER" ]; then
       ok "Claude Code already up to date ($NEW_VER)"
@@ -35,7 +43,7 @@ if command -v claude &>/dev/null; then
       ok "Claude Code updated: $CURRENT_VER → $NEW_VER"
     fi
   else
-    warn "Claude Code update failed — try manually: npm install -g @anthropic-ai/claude-code@latest"
+    warn "Claude Code update failed — try manually: ${UPDATE_CMD[*]}"
   fi
 else
   warn "Claude Code not found — install first with: make install"