Add `--json` output flag to `commit-echo history`

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Task summary

Add a --json flag to the commit-echo history command so the style profile and recent commits can be output as JSON for scripting, CI pipelines, and tool integration.

Why this is a good first issue

The task is well-scoped: it only modifies one command file and the CLI entry point. The profile data structure (StyleProfile) is already defined in src/types.ts, and the human-readable formatting already exists in formatProfile(). Adding a JSON output path is a straightforward parallel implementation.

Suggested files or areas

• src/commands/history.ts — add --json flag handling and JSON output branch
• src/index.ts — register the --json option on the history command

Acceptance criteria

• commit-echo history --json outputs a valid JSON object with profile and recentCommits keys
• commit-echo history --json exits with code 0 and output is written to stdout only (no ANSI formatting, no intro/outro)
• commit-echo history (without flag) continues to output the current human-readable format unchanged
• When no history exists, commit-echo history --json returns { "profile": null, "recentCommits": [], "totalCommits": 0 }
• npm run build succeeds with no errors

Extra context

• StyleProfile and CommitEntry types are in src/types.ts
• loadEntries and buildProfile are in src/history/store.ts
• Use JSON.stringify for output—avoid logging through @clack/prompts when --json is set