feat(alias): implement alias execution and builtin name protection

[lognarly]

Summary

Implements runtime alias expansion so user-defined shorthand commands are resolved to their full pup command before clap parses the argument list, and adds builtin name protection to prevent aliases from shadowing built-in commands.

Changes

• Added apply_expansion and expand functions to src/commands/alias.rs to rewrite args before clap sees them
• Updated src/main.rs to call commands::alias::expand after extension interception and switch cmd.get_matches() to cmd.get_matches_from(&args)
• Added builtin guard in apply_expansion (src/commands/alias.rs:82) and set (src/commands/alias.rs:55) to reject aliases that conflict with built-in commands
• Added docs/ALIASES.md covering storage locations, commands, expansion behavior, and examples
• Updated CLAUDE.md documentation index to reference docs/ALIASES.md

Testing

• test_apply_expansion_simple — verifies basic alias token replacement
• test_apply_expansion_preserves_trailing_args — verifies extra args are appended after expansion
• test_apply_expansion_preserves_leading_flags — verifies global flags before alias are retained
• test_apply_expansion_no_match — verifies args are unchanged when no alias matches
• test_set_rejects_builtin_name — verifies set returns an error for builtin command names

Related Issues

N/A

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