Plan mode: allow inline back-and-forth commenting on the plan (Google Docs style)

Problem
Plan mode already lets me attach inline comments to specific parts of the plan — great. What's missing is the back-and-forth: the agent can't reply to a comment in-thread, and I can't reply back to its reply. Every comment is effectively a one-shot note that only lands when I end my turn and the agent processes everything together. For non-trivial plans this is painful — I either approve plans I'd rather iterate on, or I burn turns re-explaining context in the main chat instead of the plan surface where the anchoring lives.

Proposal
Turn each inline comment into a real thread, Google-Docs-style:

• Agent can reply to my comment in-thread, anchored to the same selection.
• I can reply back without ending my turn — ideally live / streaming as I type, so the agent can respond to a clarifying question the moment I hit submit instead of waiting for a full plan-mode turn cycle.
• Threads are resolvable; resolved threads collapse; unresolved threads block ExitPlanMode until acknowledged.
• Agent revisions to the plan file are linked to the thread that prompted them, so I can see why a step changed.

Why it matters
Plan mode is the highest-leverage collaboration surface in Claude Code for anything non-trivial. Comments without threaded replies are a half-built version of this — the UI implies a conversation the backend doesn't support, which is more frustrating than no comments at all.