[Phase A3] Editable draft Markdown + per-section regenerate API

[terisuke]

Tracked under ADR 0002 — Phase A. Detail: implementation plan §A3.

Problem

The Markdown output textarea is readonly (static/index.html:153); the user cannot make a small inline correction without copying the text into a separate editor. Worse, when a section misses the mark, the only recourse is to regenerate the entire draft (slow on gemma4:31b, and risks losing acceptable sections). Issue #11 (style threshold) is more painful than it should be because of this.

The Preview tab also has no Copy button — switching tabs to copy is unnecessary friction.

Scope

• Make <textarea id=\"markdown-output\"> writable. Live-sync edits into the rendered preview via marked.
• Add a Copy button to the Preview tab as well.
• Add a "Regenerate this section" button that activates when the cursor is inside a ## heading subtree.
• New endpoint POST /api/drafts/{id}/regenerate-section accepts { section_anchor, brief_id, persona_id, format_id } and returns a candidate replacement for that section only.
• Replacement is presented as a diff/preview; the user accepts or rejects.

Acceptance criteria

• Editing the Markdown textarea immediately updates the preview.
• Regenerating section "## 実装" replaces only that subtree of the Markdown; other sections remain byte-identical.
• Copy works from both tabs.
• Regenerate-section call respects persona + format strategy (depends on Phase B for full effect; on Phase A this can fall back to the current single-format path).
• Unit tests for the section anchor parser (Markdown heading splitter).

Out of scope

• Visual diff UI for the regenerated section (text-only is fine for now).
• Streaming of the regenerated section (covered indirectly by Phase A2 if endpoint supports SSE).

[terisuke]

Implemented and merged via PR #51 (merge commit 81558d8). Validation recorded in docs/validation/issue-19-section-regeneration-2026-05-02.md: Evo X2 Tailnet section regeneration 167.33s, lightweight verification 23.14s, score 72.1, verification PASS, non-target prefix/suffix unchanged.