feat: render comment & activity text as formatted Markdown, and as a readable timeline

[BlueLabelscotch]

Problem

harness list pr_comment <repo>/<pr> and harness list pr_activity <repo>/<pr> both render through the generic table formatter (handler_type: endpoint, plain columns: [...]).

Desired behavior

• Comment/activity text should be rendered through the same lightweight Markdown-to-ANSI renderer already added for get pr's description (console.RenderMarkdown, in
  pkg/console/markdown.go) — bold, inline code, links, headers, lists should render styled, not as raw syntax.
• Add a dedicated text/timeline view — e.g. harness get pr:activity <repo>/<pr> — that prints activity chronologically (oldest-first or newest-first, TBD), one entry per block (author,
  relative timestamp, full rendered comment body), with threaded replies (via parent_id/order+sub_order) indented under their parent instead of shown as sibling rows. This mirrors the
  pattern already used for get pr:insight (modules/code/insight.go — best-effort section fetch + dedicated text formatter).
• Table output (list pr_comment, list pr_activity) can stay as-is for scripting/machine use (--format json/csv/table), but the default human-facing text path should go through the new
  renderer/timeline rather than a table dump.