[FEATURE]: Export artifacts from a sprint iteration

Add the ability to export the artifacts produced during a sprint iteration (requirements, user stories, design decisions, tasks, code) in various formats and to external tools.

Use cases:
- Export a full sprint's artifacts as a Markdown bundle — useful for archiving, sharing in PRs, or feeding into other AI tools
- Export to Confluence — push requirements, design docs, and user stories as structured Confluence pages
- Export to Jira — sync user stories and tasks as Jira issues with proper hierarchy (epic → story → subtask)

The issue names no files, tests, or entry points. Start by mapping how sprint artifacts are stored and whether any export path already exists; then define the Markdown, Confluence, and Jira boundaries. Done means the requested artifact types can be exported in the stated formats and synchronized with the external tools, including Jira hierarchy.

Written by the indexing model from the issue text.