refactor: flatten docs/ — move V2 docs from docs/v2/ to docs/

- git mv docs/v2/*.md docs/ (11 planning documents)
- Remove empty docs/v2/ directory
- Update all references: AGENTS.md, CHANGELOG.md, README.md, ARCHITECTURE.md
- Fix outdated repo tree in ARCHITECTURE.md to match actual structure

CHANGELOG.md

@@ -3,15 +3,15 @@
 ## [Unreleased] — V2 Planning Complete (2026-06-29)
 
 ### Added (V2 Planning Documents)
-- `docs/v2/PROGRESS.md` — Status tracker for V2 development
-- `docs/v2/ARCHITECTURE.md` — System architecture (GoFiber + React + Electron)
-- `docs/v2/API.md` — REST API + WebSocket specification
-- `docs/v2/DATA_MODELS.md` — Extended data models (Groups, PortForward, Workspace)
-- `docs/v2/UI_COMPONENTS.md` — React component tree + state management
-- `docs/v2/UI_TERMIUS_REFERENCE.md` — Termius visual reference
-- `docs/v2/SPRINT_PLAN.md` — Sprint-by-sprint task breakdown
-- `docs/v2/BUILD_SYSTEM.md` — Cross-platform build pipeline
-- `docs/v2/PERFORMANCE.md` — Performance & stability guide
+- `docs/PROGRESS.md` — Status tracker for V2 development
+- `docs/ARCHITECTURE.md` — System architecture (GoFiber + React + Electron)
+- `docs/API.md` — REST API + WebSocket specification
+- `docs/DATA_MODELS.md` — Extended data models (Groups, PortForward, Workspace)
+- `docs/UI_COMPONENTS.md` — React component tree + state management
+- `docs/UI_TERMIUS_REFERENCE.md` — Termius visual reference
+- `docs/SPRINT_PLAN.md` — Sprint-by-sprint task breakdown
+- `docs/BUILD_SYSTEM.md` — Cross-platform build pipeline
+- `docs/PERFORMANCE.md` — Performance & stability guide
 
 ### Architecture Decision
 - Backend: GoFiber v2 (fasthttp-based) replacing chi/net/http