feat: add --format json flag to all read commands

[hanw39]

Context

BlogWatcher is intended to be used by AI agents via shell commands. Currently all output is human-readable colored text, which is fragile for programmatic parsing.

Proposed Change

Add --format json (short: -f json) flag to: articles, blogs, categories, stats (new).

Success output:

{ "data": [ ... ] }

Error output (to stdout, non-zero exit):

{ "error": "message" }

Default behavior (no flag) is unchanged.