[3/7] Markdown & JSON output for N-level headings

[shouze]

Context

src/output.ts buildMarkdownOutput currently hardcodes ## ${group.sectionLabel} with no depth concept. buildJsonOutput has no section/team information at all today (flat results[]). ReplayOptions / buildReplayCommand serialize --group-by-team-prefix as a flat comma-separated string.

Part of EPIC #125. Depends on #177, #178.

Solution

• Update buildMarkdownOutput to emit heading depth derived from the tree level from [1/7] Data model & core hierarchical grouping algorithm #177 (##, ###, ####, ...), capped at a sane max (e.g. H6).
• Design and implement a JSON output shape carrying the hierarchy (nested sections tree, or a flat list with level/path per repo - decide and document as a breaking change).
• Update ReplayOptions / buildReplayCommand to serialize the chain syntax (/ for depth, , for independent chains) and the consolidation flag from [2/7] Advanced consolidated rendering #178.
• Keep docs/usage/team-grouping.md markdown example blocks in sync (full doc rewrite is issue v1.0.4 #7, but keep code samples correct here to ease review).

Acceptance Criteria

• A 2-level grouped result renders ## for the top level and ### for the nested level in markdown output.
• A 3-level grouped result renders up to ####.
• JSON output exposes the hierarchy in a documented, stable shape.
• The replay command reproduces the exact chain + consolidation flags used for the original run.

Definition of Done

• Full src/output.test.ts coverage: 2/3-level markdown headings, JSON shape, replay command round-trip.
• 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 and [2/7] Advanced consolidated rendering #178 being merged first.