Thread vault_paths config through all curaitor skills

[jdidion]

Goal

Make all Obsidian page/folder locations configurable via `user-settings.yaml` → `vault_paths` (added in #12). Skills currently hardcode paths — this issue tracks threading the config through each skill.

Defaults (from #12)

• `journal_club`: `Journal Club Ideas.md` (vault root)
• `bookmarks`: `Bookmarks.md`
• `tools_and_projects`: `Tools & Projects.md`
• `curaitor_root`: `Curaitor`
• `library_root`: `Library`
• `topics_root`: `Topics`
• `recycle_file`: `Curaitor/Recycle.md`

Skills that need updating

• `cu:read` — reads `Curaitor/Inbox/`, writes to `Library/`, `Tools & Projects.md`, `Bookmarks.md`, `Curaitor/Recycle.md`
• `cu:review` — reads `Curaitor/Review/` or `Curaitor/Ignored/`, writes to `Curaitor/Inbox/`, `Library/`, `Topics/`, `Curaitor/Recycle.md`, `Bookmarks.md`, `Tools & Projects.md`
• `cu:triage` — writes to `Curaitor/Inbox/`, `Curaitor/Review/`, `Curaitor/Ignored/`, `Curaitor/Recycle.md`
• `cu:discover` — writes to `Curaitor/Inbox/`, `Curaitor/Review/`, `Curaitor/Ignored/`, `Curaitor/Recycle.md`
• `cu:review-ignored` — reads `Curaitor/Ignored/`
• `cu:status` — reads `Curaitor/*/`, `Curaitor/Recycle.md`

Scripts that need updating

• `scripts/triage-write.py` — hardcodes `Curaitor/Inbox`, `Curaitor/Review`, `Curaitor/Ignored`, `Library`, `Curaitor/Recycle.md`
• `scripts/prefetch-review.py` — hardcodes `Curaitor/Review`, `Curaitor/Ignored`, `Curaitor/Inbox`
• `scripts/accuracy-metrics.py` — hardcodes `Curaitor/*`, `Curaitor/Recycle.md`

Approach

1. Add a single loader in a shared utility (e.g., `scripts/_paths.py`) that reads `user-settings.yaml` `vault_paths` with fallback to defaults.
2. Skills reference paths via `{{vault_paths.curaitor_root}}/Inbox` in their docs, and scripts import the loader.
3. New `/cu:journal-club` skill (not yet built) reads `{{vault_paths.journal_club}}` directly.