Support reply threading with issue comment --reply-to

Context

Issue #10 implements tangled issue comment for adding top-level comments to an issue. The sh.tangled.repo.issue.comment lexicon includes an optional replyTo field (an AT-URI) that supports threading replies to specific comments, but issue #10 deferred this to keep scope manageable.

Comment threading is useful for focused back-and-forth discussions within an issue — for example, replying directly to a reviewer's question rather than posting an unthreaded follow-up.

Tasks

• Add --reply-to <comment-number> option to tangled issue comment <issue-number>.
  • Resolve the comment number to its AT-URI (list comments for the issue, then pick by index).
  • Set the replyTo field in the sh.tangled.repo.issue.comment record to the resolved AT-URI.
• Update tangled issue view to display the thread structure visually — replies should be shown indented under the comment they reply to.
• Add tests covering the --reply-to flag and threaded display in issue view.

Notes

Depends on issue #10 (tangled issue comment base implementation). The replyTo field is optional in the lexicon, so existing comments without it remain unaffected.