Classify referenced files by functionality through /group #12

Parent User Story: As a student exploring an unfamiliar repository, I want OpenCode to group files by functionality and explain how the groups relate, so that I can understand the repository’s structure.

Estimated effort: 8 hours
Size: M
Dependencies: Future Learn Mode integration in #15
Milestone: Sprint 1

Scope update:

The original design grouped session-referenced files by artifact role. The final design provides a repository-wide learning map, while using files read in the current session to prioritize relevant groups.

Implementation:

• feat(tui): group files based on functionality for repository learning #18 adds grouping, subgroups, navigation, and file pages.
• feat(tui): add context-aware repository group explanations #24 adds session prioritization, model-generated names, and explanations.
• test(tui): expand repository map coverage #25 expands unit and integration test coverage for repository grouping, prioritization, explanation prompts, and fallback behavior.
• test(tui): verify group map navigation and explanations #29 adds end-to-end tests for /group navigation and group/subgroup explanation requests in Learn mode.

Acceptance criteria:

• /group opens an interactive Repository Learning Map.
• Files are grouped without project-specific hardcoded mappings.
• The map supports up to 30 groups and 20 subgroups per group.
• Session-referenced groups are prioritized.
• Users can explain a group or subgroup.
• Explanations include file evidence, relationships, a reading order, and a learning question.
• Empty or unfamiliar repositories do not cause crashes.
• Automated tests cover grouping, limits, prioritization, prompts, and fallback behavior.