feat: add /add-template command for registering custom LaTeX templates (#30)

Users could already swap the stock moderncv/cover.cls templates, but only by
hand-editing the guidance in 05-cv-templates.md and 06-cover-letter-templates.md.
/add-template automates that:

- Interviews the user for the template's instructions: compile engine, fonts
  (bundled files or system), style rules to preserve, and hard page limit
- Stores the template profile-agnostic ([PLACEHOLDER] tokens) under templates/
  with a TEMPLATE.md manifest, so templates are safe to commit and share
- Runs a mandatory test compile with dummy data before registering anything
- Activates via a single managed block in 05/06, which /apply already reads,
  so no changes to the /apply workflow are needed; --use default is a clean
  revert to the stock templates
- --list and --use <name> manage multiple registered templates

Docs: README (commands list, file structure, LaTeX templates section) and
SETUP.md (compile section pointer).

SETUP.md

@@ -151,6 +151,8 @@ cd cv && lualatex main_<company>.tex && cd ..
 cd cover_letters && xelatex cover_<company>_<role>.tex && cd ..
 ```
 
+These commands apply to the stock templates (moderncv CV, `cover.cls` cover letter). If you'd rather use your own LaTeX template, run `/add-template` — it captures the template's compile engine, fonts, style rules, and page limit, test-compiles it, and wires it into `/apply`. See the "LaTeX templates" section in the README.
+
 ## Troubleshooting
 
 ### "salary_data.json not found"