[2/7] Advanced consolidated rendering

[shouze]

Context

Issue #125's third proposal asks for a more compact rendering mode: instead of one heading per nesting level, collapse chains where a parent has a single meaningful branch into one heading with a summary suffix, e.g. gamme-lead-client (including p1, unset, etc.) instead of separate ##/### headings. This builds directly on the tree produced by #177.

Part of EPIC #125. Depends on #177.

Solution

• Add a pure consolidation function (in src/group.ts or a new src/consolidate.ts) that walks the tree from [1/7] Data model & core hierarchical grouping algorithm #177 and collapses single-branch-per-level chains into one node, synthesizing a label like "<parent> (including <child1>, <child2>, ...)".
• Define and implement the activation mechanism: a dedicated CLI flag (name TBD, e.g. --group-by-team-prefix-consolidate) or a heuristic threshold - finalize and document the chosen approach in the PR.
• Keep the underlying tree structure intact for JSON output (issue chore release v1.0.2 #3) even when the markdown rendering consolidates; document how the two interact.

Acceptance Criteria

• A tree with a single child at every level below the root is collapsed into one heading with an "(including ...)" suffix listing the collapsed branch labels.
• A tree with multiple children at some level is NOT collapsed at that level (only chains that are unambiguous are consolidated).
• The activation flag/mode is off by default unless decided otherwise in the PR (document the decision).

Definition of Done

• Unit tests for the consolidation function, isolated from rendering concerns.
• bun test, bun run lint, bun run format:check, bun run knip, bun run build.ts all green.
• Depends on [1/7] Data model & core hierarchical grouping algorithm #177 being merged first.