feat(install-plugins): interactive ctx7 login + auto-setup, ignore find-docs

STEP 6 ctx7 auth, when anonymous:
- interactive TTY -> prompt [y/N] then run `ctx7 login`; non-interactive
  (CI/headless/re-run) keeps text guidance, never opens a browser or blocks.
- run `ctx7 setup --claude --cli` when the find-docs skill is absent, to
  (re)install CLI+Skills mode. Guarded on absence so a re-run never clobbers
  a customized ~/.claude/rules/context7.md.

gitignore skills/find-docs/: it is a ctx7-managed skill materialized by
`ctx7 setup --claude --cli` into ~/.claude/skills (symlink to repo skills/),
re-created on demand by Step 6 — not vendored here.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01C8bmCXTHNccS7KRV4gWXEF

.gitignore

@@ -69,6 +69,11 @@ skills/frontend-design
 skills/darwin-skill
 skills/find-skills
 
+# Context7 docs-lookup skill — installed by `ctx7 setup --claude --cli`
+# (install-plugins.sh Step 6, when absent) into ~/.claude/skills (a symlink to
+# this repo's skills/). ctx7-managed and re-created on demand — not vendored here.
+skills/find-docs/
+
 # Staging area used by lib/toggle-external.sh when disabling a tool
 skills-disabled/