feat: add claude-usage CLI tool with Fish shell aliases

Implements a fast Bun-based CLI utility to check Claude API rate limit usage by reading OAuth credentials and making a minimal API request to extract rate limit headers.

Provides three Fish shell aliases (cu, ccu, usage) for convenient access to the claude-usage command.

home/dot_config/fish/functions/ccu.fish

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+function ccu --description 'Check Claude usage (alias for claude-usage)'
+    claude-usage $argv
+end

home/dot_config/fish/functions/cu.fish

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+function cu --description 'Check Claude usage (alias for claude-usage)'
+    claude-usage $argv
+end

home/dot_config/fish/functions/usage.fish

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+function usage --description 'Check Claude usage (alias for claude-usage)'
+    claude-usage $argv
+end