owner-led(visual-agent): visual-change detection + affected-route mapping

[JSONbored]

Parent roadmap: #525
Parent epic: #577

Owner-led — assigned to @JSONbored.

Goal

Visual-change detection + affected-route mapping: classify a PR as visual when it touches apps/gittensory-ui/**, *.tsx, *.css, or other UI paths, and map changed files → the routes/pages to screenshot.

Reuse

PR file-diff fetch from the submission-gate pattern; the UI route table in apps/gittensory-ui.

Scope

• Path globs for "visual" detection (configurable).
• Changed-file → affected-route resolver (route map or heuristic); fall back to a default route set.
• Skip non-visual PRs cleanly (no comment).

Acceptance

Given a PR diff, returns the correct visual/non-visual verdict + the list of routes to capture.