Add CSV output to dispatch stats

[jongio]

Problem

dispatch stats prints a readable summary and --json for scripts, but there is no format that drops straight into a spreadsheet or a plotting tool. The per-repo, per-branch, and per-host breakdowns and the --calendar day counts are exactly the kind of tabular data people want to chart, and JSON makes that an extra parsing step for a quick pivot table.

flowchart TD
    A[dispatch stats --csv] --> B[Build stats report]
    B --> C[Emit section,label,count rows]
    A2[dispatch stats --csv --calendar] --> D[Add date,count,intensity rows]
    C --> E[CSV to stdout]
    D --> E

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Proposed solution

Add a --csv output format to dispatch stats:

• Emit the repository, branch, and host-type breakdowns as CSV rows with a section,label,count shape, using encoding/csv so labels with commas or quotes are escaped correctly.
• Include a totals row (sessions, turns, files) so the summary numbers are present too.
• When combined with --calendar, append the per-day series as date,count,intensity rows under a calendar section.
• Reject combining --csv with --json, since they are two output formats.

Honor the existing --repo, --branch, --folder, --since, and --until filters exactly as the text and JSON paths do.

Acceptance criteria

• dispatch stats --csv writes the totals and the repo/branch/host breakdowns as valid CSV.
• dispatch stats --csv --calendar appends the per-day counts and intensity.
• The filter flags narrow the CSV the same way they narrow text and JSON output.
• Labels containing commas or quotes are escaped per RFC 4180 (via encoding/csv).
• Passing both --csv and --json is rejected with a clear error.
• Help text documents --csv; unit tests cover the breakdowns, the calendar rows, and the conflicting-flag case.

Notes

buildStatsReport and buildActivityCalendar already produce the structured data; this change only adds a renderer next to writeStatsJSON/writeStatsText.