skills/book-gig: accumulate multi-batch weekend run logs into consolidated cumulative reports

[JoshuaVSherman]

Child of web-jam-tools#875 ("book-gig skill phase 2 enhancements (Epic)").

What this builds

Enhances writeDropboxRunLog in src/book-gig/gmail.ts and src/book-gig/html.ts so when multiple outreach batches are dispatched for the same target weekend across separate invocations, the run log (book-gig-run-<weekend>.md) and Dark Mode HTML artifact (book-gig-run-<weekend>.html) accumulate and merge all dispatched candidate venues into a single cumulative weekend report rather than overwriting with only the latest batch.

1. Reads any existing run log for the target weekend slug if present.
2. Merges candidate venue rows, pitch cards, and batch dispatch tallies (requested, sent, skipped), deduplicating by venueId.
3. Re-renders the consolidated Markdown log and standalone Dark Mode HTML artifact.
4. Adds unit tests in test/book_gig.test.ts verifying multi-batch accumulation and deduplication.

Acceptance criteria

1. Dispatching a second batch of candidates for the same weekend updates the weekend run log to include all candidates from both batches.
2. Batch dispatch metrics report cumulative requested and sent counts.
3. deno task test, deno task fmt:check, deno task lint, and deno task sast pass 100% green.