[7/7] Documentation & changelog

[shouze]

Context

docs/usage/team-grouping.md fully documents the current flat single-level algorithm, TUI section rows, pick mode, --pick-team, re-pick/undo, and the team cache. It needs a substantial rewrite once the hierarchical behavior lands. docs/reference/cli-options.md and CHANGELOG.md also need updates, plus a required blog post for the major release.

Part of EPIC #125. Depends on #182.

Solution

• Rewrite docs/usage/team-grouping.md: hierarchy syntax (/ and ,), nesting examples, consolidated rendering mode, updated pick-team section covering all hierarchy levels, updated TUI text mockups.
• Update docs/reference/cli-options.md for the new --group-by-team-prefix syntax and any new flag from [2/7] Advanced consolidated rendering #178.
• Add the new row in CHANGELOG.md (major bump per AGENTS.md).
• Write docs/blog/release-vX-0-0.md and add a row to docs/blog/index.md. Ask the user interactively for the highlights and description before writing - do not draft this from code alone.

Acceptance Criteria

• docs/usage/team-grouping.md accurately describes the shipped behavior end to end, with working examples.
• docs/reference/cli-options.md reflects the final CLI syntax.
• CHANGELOG.md has no _pending_ rows for this release.
• Blog post published with user-provided highlights.

Definition of Done

• bun run docs:build succeeds locally.
• Depends on [6/7] CLI wiring end-to-end #182 being merged first (final CLI/output behavior must be locked before finalizing docs).