docs(v3): add Mission Control redesign planning docs

v3 architecture planning: task plan with core concept, user requirements,
and architecture questions for adversarial review. Findings doc with
codebase reuse analysis (keep/replace/drop). Progress log for v3 sessions.

docs/v3-findings.md

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+# BTerminal v3 — Research Findings
+
+## Current Codebase Reuse Analysis
+
+### Can Reuse (with modifications)
+- **Agent session infrastructure**: AgentPane, agent store, agent dispatcher, SDK messages adapter, sidecar bridge — core agent functionality stays
+- **Terminal infrastructure**: TerminalPane, pty-bridge — terminal rendering unchanged
+- **Markdown rendering**: MarkdownPane, highlight utils — markdown viewer stays
+- **Sidecar management**: SidecarManager, agent-runner.mjs — backend agent orchestration
+- **PTY management**: PtyManager — backend terminal management
+- **Session persistence**: session-bridge, SessionDb — needs schema extension for projects
+- **Theme system**: theme store, catppuccin CSS — visual layer unchanged
+- **Notification system**: toast notifications — stays as-is
+- **Claude bridge**: profiles + skills — reusable per project
+- **StatusBar**: needs redesign but concept stays
+
+### Must Replace
+- **Layout store**: Current pane-based grid system → project-based workspace system
+- **TilingGrid**: CSS Grid tiling → project box layout with internal structure
+- **SessionList sidebar**: Pane list → project/group navigation
+- **App.svelte**: Root layout completely changes
+- **Settings dialog**: Per-app settings → per-project settings
+
+### Can Drop
+- **Detached pane mode**: Doesn't fit the workspace model
+- **Drag-resize splitters**: Project boxes have fixed internal layout
+- **Layout presets (1-col, 2-col, etc.)**: Replaced by N-project horizontal layout
+- **Remote machine integration**: Defer to v4 (complexity too high to combine)
+- **ctx ContextPane**: Becomes a workspace tab instead of a pane type
+
+---
+
+## Adversarial Agent Findings
+
+(To be filled by adversarial review agents)