fix(install-plugins): auth-aware ctx7 guidance + drop obsolete MCP nudge

Step 6 printed its ctx7 hints unconditionally — telling already-authed users
to log in, and pointing at `ctx7 setup --claude` (MCP-adjacent). Anonymous mode
is fully functional (docs + library work without auth); auth only buys rate
limits, so setup was never required for ctx7 to work.

- Detect auth via an offline oracle: credentials.json presence (XDG-aware),
  no subprocess / network / browser — mirrors the idempotent-claude fix.
- Authenticated -> "ctx7 authenticated"; anonymous -> non-blocking guidance
  (works anonymously; `ctx7 login`, `--no-browser` for headless). The installer
  guides, never launches login/setup.
- Drop `ctx7 setup --claude`: leftover reopening MCP path, aligns w/ TODO:48
  CLI-only decision.

Verified: bash -n, shellcheck (no new findings), + simulated both auth states
(credentials.json present/absent) — correct branch each, credentials restored.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VeBXkDr74N9whdiJyjzyVN

install-plugins.sh

@@ -573,11 +573,19 @@ else
     err "ctx7 install failed — run manually: npm install -g ctx7"
   fi
 fi
-# Suggest setup for Claude Code integration (optional — ctx7 also works standalone)
+# ctx7 auth — detect + guide only (an installer must never open a browser / log you in)
 if command -v ctx7 &>/dev/null; then
-  info "Run 'ctx7 setup --claude' to configure Context7 for Claude Code"
-  info "Or use ctx7 standalone: ctx7 docs /vercel/next.js \"middleware\""
-  info "Free higher rate limits: ctx7 login (OAuth) or --api-key from context7.com/dashboard"
+  # Deterministic offline oracle: ctx7's OAuth token lives here (XDG-aware).
+  # Present => authenticated; absent => anonymous. No subprocess, no network, no browser.
+  ctx7_creds="${XDG_CONFIG_HOME:-$HOME/.config}/context7/credentials.json"
+  if [ -f "$ctx7_creds" ]; then
+    ok "ctx7 authenticated (full rate limits)"
+  else
+    info "ctx7 works anonymously — docs + library already usable, no auth required."
+    info "For higher rate limits, authenticate:  ctx7 login   (opens a browser)"
+    info "  headless:  ctx7 login --no-browser   (prints a URL to open yourself)"
+  fi
+  info "Standalone usage:  ctx7 docs /vercel/next.js \"middleware\""
 fi
 
 # ============================================================