refactor(fiction-assistance)!: drop line-editor agent, add proofread and check-consistency commands

Two shifts in scope:

- Remove the line-editor agent. The plugin no longer mutates the
  manuscript — all commands report findings to the conversation
  and leave revisions to the writer.

- Consolidate the consistency surface. /check-voice and
  /check-continuity were narrow checks with overlapping concerns;
  they are replaced by /check-consistency, a single broader pass
  covering author voice, character voice, words-vs-actions
  alignment, and in-world believability. Plot-logic and
  sequencing concerns formerly in /check-continuity are now
  served by /generate-timeline's consistency-issues section.

- Add /proofread for the mechanical layer that previously had no
  command equivalent — grammar, tense, punctuation, spelling,
  agreement, sentence-level clarity. Reports issues; does not edit.

Command count unchanged at nine. The plugin is now agent-free and
mutation-free; every command is read-only on the manuscript.

BREAKING CHANGE: /check-voice and /check-continuity are removed
(superseded by /check-consistency). The line-editor agent is
removed (use /proofread for reporting; the writer makes the edits).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -28,7 +28,7 @@
     {
       "name": "fiction-assistance",
       "source": "./fiction-assistance",
-      "description": "General-purpose fiction-writing guidance and analysis — story stats, continuity and voice checks, representation tests, developmental and strategic review, world-building extraction, timeline construction, and a line-editor agent."
+      "description": "General-purpose fiction-writing guidance and analysis — proofreading, consistency and believability, story stats, representation tests, developmental and strategic review, world-building extraction, timeline construction, final-review synthesis. Reports only; never edits."
     }
   ]
 }