Add ctx stats command showing store summary

[selwyntheo]

Add a new ctx stats command that displays a summary of the context store.

Output should include:

• Total number of packs
• Total number of blobs
• Total store size on disk
• Date of oldest and newest pack

Acceptance criteria:

• ctx stats prints a human-readable summary
• Works correctly on an empty store (shows zeros)
• Add a test case

Files to look at:

• cmd/ctx/commands.go — add the new command
• internal/store/ — store access utilities