fix(airdrop): scope _determine_tier to org merged PRs (fixes #8184)

[jjb9707]

Summary

_determine_tier() decided an airdrop claimant's tier — up to CORE = 200 wRTC — from GitHub activity queried with author:<user> merged:true against /search/commits. That counts the user's commits anywhere on GitHub, so any established account reached CORE without ever contributing to RustChain (see #8184).

Root cause

• Query was /search/commits?q=author:<user> merged:true — no org:/repo: scope, and merged:true is a PR-search qualifier sent to the commits endpoint.
• Result: total_prs reflected "is this a real GitHub user with history", not "did they contribute here".

Fix

Switch to the issues search scoped to the org and to merged PRs:

contrib_resp = requests.get(
    "https://api.github.com/search/issues",
    headers=headers,
    params={
        "q": f"author:{github_username} org:Scottcjn is:pr is:merged",
        "per_page": 1,
    },
    timeout=10,
)

The tier now reflects actual RustChain contributions, paired with the airdrop's existing per-account/per-wallet dedup.

Verification

• Updated test_eligibility_with_mock_github to match the new /search/issues endpoint.
• Added test_determine_tier_scoped_to_org regression test asserting the query is scoped (org:Scottcjn, is:pr, is:merged) and no longer uses the unscoped merged:true qualifier, and that 5 org-merged PRs => CORE.
• Full suite python -m unittest test_airdrop_v2 — 32 passed.

Fixes #8184.

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