fix(repo-stats): split Slack payload body into ≤3000-char section blocks

[Copilot]

The weekly repo-stats workflow was failing when posting to Slack because the full report was stuffed into a single section block, which Slack caps at 3000 characters.

Changes

• awk chunker — splits slack_report.txt line-by-line into numbered temp files (/tmp/sw_chunk_N.txt), flushing a new file whenever the next line would push the current chunk past 3000 chars
• Dynamic jq payload builder — loops over chunk files to build --rawfile c${i} args and corresponding section-block expressions, then injects them into the Block Kit payload alongside the fixed header/context/divider blocks
• Guard rail — NUM_CHUNKS is validated as a positive integer before the loop, defaulting to 1 on unexpected output from awk

# one section block per chunk, all under 3000 chars
SECTION_EXPRS+=("{\"type\":\"section\",\"text\":{\"type\":\"mrkdwn\",\"text\":\$c${i}}}")

Pure bash solution — awk + jq only, no Python.

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