[codex] Validate template input and project paths

[cgdusek]

Summary

• honor absolute destination paths instead of re-rooting them under process.cwd()
• accept documented beginner template aliases like token and asset
• fail fast for unknown template values instead of scaffolding an empty project

Why

Issue #3 reproduces in the current CLI:

• absolute paths like /tmp/my-app are treated as relative to the current checkout
• invalid --template values still report success after creating an empty scaffold
• the help text advertises template values that do not align with the shipped beginner templates

Impact

• scripted and non-interactive usage can target absolute paths safely
• documented template names now work as expected
• invalid template input is rejected early instead of generating a broken starter

Validation

• node /Users/charlesdusek/Code/create-canton-app/bin/create-canton-app.js --help
• env PATH="$HOME/.dpm/bin:$HOME/n/bin:/usr/bin:/bin:/usr/sbin:/sbin" node /Users/charlesdusek/Code/create-canton-app/bin/create-canton-app.js /tmp/pr2-verify/abs-token --template token
• verified /tmp/pr2-verify/abs-token/daml.yaml contains name: abs-token
• env PATH="$HOME/.dpm/bin:$HOME/n/bin:/usr/bin:/bin:/usr/sbin:/sbin" node /Users/charlesdusek/Code/create-canton-app/bin/create-canton-app.js invalid-template --template nope
• verified the invalid template run exits non-zero and creates no project

Closes #3.