Improve error messages for common mistakes

[selwyntheo]

Several commands could have friendlier error messages for common user mistakes.

Examples to improve:

• Running ctx pack without ctx init first → suggest running ctx init
• Running ctx show with an invalid hash → explain the expected format (full hash, short prefix, or ctx:// URI)
• Running ctx diff with only one argument → show usage hint
• Running ctx replay on a non-existent hash → suggest ctx log to find valid hashes

Acceptance criteria:

• Each improved error message includes a helpful suggestion
• No existing tests break
• Add test cases for at least 2 of the improved messages

Files to look at:

• cmd/ctx/commands.go — command definitions and validation
• internal/store/hash.go — hash parsing and resolution