refactor: flatten vault structure — simplify migration API and flatten demo vault

- Simplify flatten_vault API to return usize instead of MigrationResult struct
- Add KEEP_FOLDERS: attachments/ and _themes/ alongside type/, config/, theme/
- Use HashSet for collision tracking in unique_filename
- Update wikilinks from path-based [[folder/slug]] to title-based [[slug]]
- Clean up empty directories after flattening
- Flatten demo-vault-v2: move all notes from type-based subfolders to root
- Update smoke tests for flat vault structure
- Remove migrate_to_flat_vault from repair_vault (one-time migration only)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

demo-vault-v2/area-finance.md

@@ -16,7 +16,7 @@ Personal and business finances. Covers investing, savings, tax planning, and the
 
 ## Philosophy
 
-The approach to finance is intentionally simple: automate what can be automated, invest passively for the long term, and focus energy on growing business revenue rather than optimizing portfolio returns. See [[evergreen/index-funds-and-intellectual-humility]] for the underlying thinking.
+The approach to finance is intentionally simple: automate what can be automated, invest passively for the long term, and focus energy on growing business revenue rather than optimizing portfolio returns. See [[index-funds-and-intellectual-humility]] for the underlying thinking.
 
 ## Key measures
 

demo-vault-v2/area-health.md

@@ -17,7 +17,7 @@ Physical and mental health. Encompasses cycling training, gym work, nutrition, s
 
 ## Why this area matters
 
-Health is the foundation everything else is built on. Consistent training and sleep directly correlate with better creative output, clearer thinking, and more sustainable work habits. The [[evergreen/training-load-and-knowledge-work]] connection is real and measurable.
+Health is the foundation everything else is built on. Consistent training and sleep directly correlate with better creative output, clearer thinking, and more sustainable work habits. The [[training-load-and-knowledge-work]] connection is real and measurable.
 
 ## Key measures
 

demo-vault-v2/area-learning.md

@@ -10,15 +10,15 @@ Continuous learning through reading, courses, conversations, and writing. The in
 ## Scope
 
 - **Reading** — books, articles, newsletters, research papers. Target of 20+ books per year
-- **Writing as learning** — [[evergreen/the-compound-effect-in-knowledge-work]] captures why writing notes and evergreen pieces accelerates understanding
+- **Writing as learning** — [[the-compound-effect-in-knowledge-work]] captures why writing notes and evergreen pieces accelerates understanding
 - **Conversations** — podcast guests, conference hallway chats, 1:1s with smart people
 - **Courses & talks** — occasional online courses, preparing conference talks as a forcing function
 
 ## Philosophy
 
-Learning is not about consuming more — it's about extracting and connecting ideas. The [[procedure-evergreen-note-writing]] process exists to turn raw input into lasting, reusable knowledge. Every book note in [[note/]] should eventually produce at least one evergreen idea.
+Learning is not about consuming more — it's about extracting and connecting ideas. The [[procedure-evergreen-note-writing]] process exists to turn raw input into lasting, reusable knowledge. Every book note in the vault should eventually produce at least one evergreen idea.
 
-See [[evergreen/reading-more-by-reading-better]] for the approach to reading.
+See [[reading-more-by-reading-better]] for the approach to reading.
 
 ## Key measures
 

demo-vault-v2/area.md

@@ -0,0 +1,7 @@
+---
+Is A: Type
+---
+
+# Area
+
+An Area is an ongoing sphere of responsibility in your life or work — something you maintain indefinitely. Areas have standards to uphold, not completion dates.

demo-vault-v2/dark.md

@@ -0,0 +1,54 @@
+---
+Is A: Theme
+Description: Dark variant with deep navy tones
+background: "#0f0f1a"
+foreground: "#e0e0e0"
+card: "#16162a"
+popover: "#1e1e3a"
+primary: "#155DFF"
+primary-foreground: "#FFFFFF"
+secondary: "#2a2a4a"
+secondary-foreground: "#e0e0e0"
+muted: "#1e1e3a"
+muted-foreground: "#888888"
+accent: "#2a2a4a"
+accent-foreground: "#e0e0e0"
+destructive: "#f44336"
+border: "#2a2a4a"
+input: "#2a2a4a"
+ring: "#155DFF"
+sidebar: "#1a1a2e"
+sidebar-foreground: "#e0e0e0"
+sidebar-border: "#2a2a4a"
+sidebar-accent: "#2a2a4a"
+text-primary: "#e0e0e0"
+text-secondary: "#888888"
+text-muted: "#666666"
+text-heading: "#e0e0e0"
+bg-primary: "#0f0f1a"
+bg-sidebar: "#1a1a2e"
+bg-hover: "#2a2a4a"
+bg-hover-subtle: "#1e1e3a"
+bg-selected: "#155DFF22"
+border-primary: "#2a2a4a"
+accent-blue: "#155DFF"
+accent-green: "#00B38B"
+accent-orange: "#D9730D"
+accent-red: "#f44336"
+accent-purple: "#A932FF"
+accent-yellow: "#F0B100"
+accent-blue-light: "#155DFF33"
+accent-green-light: "#00B38B33"
+accent-purple-light: "#A932FF33"
+accent-red-light: "#f4433633"
+accent-yellow-light: "#F0B10033"
+font-family: "'Inter', -apple-system, BlinkMacSystemFont, sans-serif"
+font-size-base: 14px
+editor-font-size: 16
+editor-line-height: 1.5
+editor-max-width: 720
+---
+
+# Dark Theme
+
+A dark theme with deep navy tones for comfortable night-time reading.