Add submarine TUI prototype with mock data

Bubble Tea prototype for browsing repositories with many nested git
submodules: collapsible tree, pins shown as ref + distance, summarized
issue words with severity levels, detail pane, async loading via a
Provider interface backed by mock data. Includes docs/ with
requirements, state model, UX guide, architecture and decision log,
plus AGENTS.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,36 @@
+# Binaries
+/submarine
+/bin/
+/dist/
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+
+# Test and coverage output
+*.test
+*.out
+coverage.*
+*.coverprofile
+
+# Go workspace files (local)
+go.work
+go.work.sum
+
+# Environment
+.env
+
+# Editors and IDEs
+.idea/
+.vscode/
+.zed/
+*.swp
+*~
+
+# OS files
+.DS_Store
+Thumbs.db
+
+# Claude Code local settings
+.claude/settings.local.json