Add gh auth preflight check to authorize-partner-secret.sh

[michieldegezelle]

Summary

Stacked on #33 (bash 3.2 fix). Two real hiccups came up while rolling this script out:

• macOS's bash 3.2 (fixed in fix: avoid associative arrays in authorize-partner-secret.sh for bash 3.2 #33)
• A stale/expired gh session — this only surfaced as HTTP 401: Bad credentials at the very last step (gh secret set), after already going through the full staging login flow to get a fresh partner token.

This adds a gh auth status preflight check right after resolving the target repo, before the script ever prompts for the sensitive api key — so a bad gh session fails fast with the fix steps instead of wasting a token/login cycle. Also documents checking gh auth status proactively in the README, and fixes a stale MARKET_REPOS table reference left over from #33's refactor.

Test plan

• Ran full script on bash 3.2 with a fake failing gh auth status — exits immediately with the gh auth login steps, before any prompt
• Ran full script on bash 3.2 with a fake succeeding gh auth status and fake silverfin/gh — proceeds through normally
• One real run against a real market repo with a real partner token, to confirm the preflight doesn't false-positive on a genuinely valid gh session

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