feat: add prologue support to chapters schema, storage, and UI (#29)

* feat: add prologue support to chapters schema, storage, and UI

The prologue is an optional high-level overview (motivation, outcome,
key changes, focus areas, complexity) that the coding agent generates
alongside chapters. It flows through the same JSON file → import →
API → frontend pipeline as chapters.

- Add Prologue types to @stage-cli/types with as-const enums
- Add nullable prologue JSON column to chapter_run table (migration 0004)
- Extend ChaptersFileSchema with optional prologue field
- Store prologue during import, serve in /api/runs/:runId/chapters response
- Add PrologueSection component displayed above chapter list
- Extract buildDiffArgs into git.ts for reuse
- Add Step 4 to SKILL.md with prologue generation instructions

* fix(web): use standard Tailwind spacing class in prologue section

Replace ml-5.5 with ml-6 for the focus area description margin.

* fix(web): render motivation and outcome independently in prologue

Each field is independently nullable — render whichever is present
rather than requiring both.

* feat(web): side-by-side prologue layout and show all focus areas

Match the hosted Stage monorepo's layout: prologue in a sticky left
sidebar (2:3 grid ratio) with chapters on the right, stacking
vertically on smaller screens.

Show all focus areas in Review Focus, not just concerns. Info-level
items now render with a blue info icon instead of being filtered out.

* fix(web): restore concerns-only filtering in Review Focus

Match the hosted Stage monorepo — only show non-info severity focus
areas in the Review Focus section.

* feat(web): add clickable file links in prologue Review Focus

Each focus area's first location renders as a clickable filename
that navigates to the Files changed tab, matching the hosted Stage
monorepo's pattern.

* feat(web): scroll to file when clicking prologue focus area links

Add scrollTo search param to the Files route. When a focus area's
file link is clicked, it navigates to Files changed and scrolls to
that specific file, matching the hosted Stage monorepo's behavior.

Author: dastratakos

packages/cli/drizzle/0004_silly_firebird.sql

@@ -0,0 +1 @@
+ALTER TABLE `chapter_run` ADD `prologue` text;