[Feature Request] Support project-level keybindings in .claude/keybindings.json

Bug Description
Title: Support project-level keybindings (.claude/keybindings.json)

Description:
Currently keybindings are only configurable globally (~/.claude/keybindings.json). It would be valuable to support project-level keybindings in .claude/keybindings.json, following the same pattern as other project-level configs:

Config Project Global
Rules .claude/rules/ ~/.claude/rules/
Commands .claude/commands/ ~/.claude/commands/
Settings .claude/settings.local.json ~/.claude/settings.json
MCP .mcp.json ~/.claude/.mcp.json
Keybindings ❌ .claude/keybindings.json ~/.claude/keybindings.json

Use case:
Different projects may benefit from custom shortcuts (e.g., a vim-heavy project vs a standard workflow). Project-level keybindings would allow teams to share consistent shortcuts via version control.

Suggested behavior:

Project keybindings merge with/override global keybindings
Same format as current keybindings.json