feat: Add threaded review comments + agent comment responses

Introduces GitHub-style threaded comments via parentCommentId self-reference.
Users and agents can reply within comment threads, and review agents receive
comment IDs so they can post targeted responses via comment-responses.json.

- Migration 0032: parentCommentId column + index on review_comments
- Repository: createReply() copies parent context, default author 'you' → 'user'
- tRPC: replyToReviewComment procedure, requestPhaseChanges passes threaded comments
- Orchestrator: formats [comment:ID] tags with full reply threads in task description
- Agent IO: readCommentResponses() reads .cw/output/comment-responses.json
- OutputHandler: processes agent comment responses (creates replies, resolves threads)
- Execute prompt: conditional <review_comments> block when task has [comment:] markers
- Frontend: CommentThread renders root+replies with agent-specific styling + reply form
- Sidebar/ReviewTab: root-only comment counts, reply mutation plumbing through DiffViewer chain

apps/server/db/schema.ts

@@ -617,11 +617,13 @@ export const reviewComments = sqliteTable('review_comments', {
   lineType: text('line_type', { enum: ['added', 'removed', 'context'] }).notNull(),
   body: text('body').notNull(),
   author: text('author').notNull().default('you'),
+  parentCommentId: text('parent_comment_id').references((): ReturnType<typeof text> => reviewComments.id, { onDelete: 'cascade' }),
   resolved: integer('resolved', { mode: 'boolean' }).notNull().default(false),
   createdAt: integer('created_at', { mode: 'timestamp' }).notNull(),
   updatedAt: integer('updated_at', { mode: 'timestamp' }).notNull(),
 }, (table) => [
   index('review_comments_phase_id_idx').on(table.phaseId),
+  index('review_comments_parent_id_idx').on(table.parentCommentId),
 ]);
 
 export type ReviewComment = InferSelectModel<typeof reviewComments>;