Replace Jinja2 prompt templating with stdlib prompt composition

## Context
Jinja2 is currently imported at service startup, which is causing local failures. It is considered overkill for current prompt needs.

## Scope
- Essays, questions, and upskilling prompt composers and prompt files.
- Remove scoped Jinja2 dependencies and mentions in documentation.

## Acceptance Criteria
- Use stdlib or simple slots for templating.
- Dynamic sections should be built in Python.
- Include tests for prompt rendering.
- No changes to the tutor_lib.agents boundary.
- Documentation must be updated.

## Risks
- Prompt drift.
- Whitespace issues.
- Missing placeholders.

## Architecture Review
Remove Jinja2; use lightweight Builder-style prompt block functions, not Template Method inheritance.