feat(specs): add editable comments and mentions

Summary

• Allow spec comment authors to edit their own comments inline, with authorization checks and validation.
• Render @name style mentions as chips in Markdown while avoiding emails, code, and links.
• Add reusable mention-enabled textarea support with caret-positioned autocomplete for new and edited comments.
• Seed demo comments that make editing and mentions easy to try locally.

Testing

• mix format
• mix compile --warnings-as-errors
• mix test test/hive/specs_test.exs test/hive_web/live/spec_live/show_test.exs test/hive_web/markdown_test.exs
• mix assets.build
• git diff --check