feat: keyboard pane resizing and vim-style navigation

[maxbeizer]

Summary

Adds keyboard-driven pane resizing and fills in missing vim-style pane navigation across all two-pane plugins.

Keyboard pane resizing (+/-)

• + grows the sidebar, - shrinks it (3 columns per keypress)
• Same clamping logic as drag-to-resize (min 20-25, max = available - 40)
• Width changes persisted to state automatically
• Works from both sidebar and detail pane contexts
• Applies to all 4 two-pane plugins: git status, file browser, conversations, workspace

Vim-style pane focus (filling gaps)

focus-right bindings for git-status and file-browser-tree contexts

• Bindings added to all 10 two-pane keymap contexts for +/-

Files changed

• internal/keymap/bindings. 24 new bindingsgo
• internal/plugins/gitstatus/update_handlers. resize in sidebar, diff, and commit previewgo
• internal/plugins/filebrowser/handlers. resize in tree and preview panesgo
• internal/plugins/conversations/plugin_input. resize in sidebar and messages panesgo
• internal/plugins/workspace/keys. resize with percentage-based width (20-60%)go
• .claude/skills/keyboard-shortcuts/SKILL. documented new shortcutsmd

Closes #142

[marcus]

Hey @maxbeizer! Starling here (AI assistant on the project). 👋

Great timing on this — keyboard pane resizing has been on the wishlist for a while (closes #142), and doing it consistently across all four two-pane plugins in one PR is the right approach. +/- is an intuitive binding that'll feel natural alongside the vim nav bindings you're also filling in.

A few things I appreciate:

• Using the same clamping logic as drag-to-resize ensures keyboard and mouse paths don't diverge over time
• Width persistence to state means it actually sticks between sessions
• Documenting in the skills shortcut file is a nice touch for discoverability
• Zero deletions on +226 lines is clean — this is purely additive

This is a focused PR with a clear scope. Flagging for Marcus's review and merge. ✦