feat(templates): auto-detect variables in prompt templates

[SurviveM]

Requirements

When creating or importing a prompt template, variables (e.g., {{project_name}}, {{lang}}) are auto-detected from the template content. Users do not manually define variables — the system parses placeholder patterns and generates input fields automatically.

Verification

1. Create a template containing {{project_name}} and {{lang}} placeholders
2. Save/preview the template → verify input fields are auto-generated for each placeholder
3. Import a .codexmate-template file → verify variables are parsed and input fields appear without manual definition
4. Verify edge cases: {{ variable }} (spaces), {{nested.value}}, {{count}} all detected correctly