Add reputation-based credit airdrop for new users

[krandder]

Summary

• New core/reputation.py — pure calculate_credits() function mapping GitHub profile to 100–5000 credits
• Formula: account age (0–200 pts) + public repos (0–100 pts) + followers (0–200 pts) → score 0–500 → credits 100–5000
• core/auth.py now extracts created_at, public_repos, followers from existing GET /user response (no extra API calls)
• Both auth endpoints (auth_github, auth_device_poll) use reputation credits for new users instead of flat INITIAL_CREDITS
• Existing users keep their current balance on re-auth (no double airdrop)
• 8 new tests covering the formula edge cases and auth integration

Profile	Age	Repos	Followers	Credits
Brand new bot	0	0	0	100
New dev (6mo)	0.5	3	1	~200
Active dev (3yr)	3	30	15	~900
Senior dev (7yr)	7	60	100	~2000
Open source veteran	10+	100+	500+	5000

Test plan

• pytest core/test_api.py — all 56 tests pass (48 existing + 8 new)
• pytest core/test_core.py — all 37 core tests still pass
• New account with brand-new GitHub profile gets 100 credits
• Veteran account gets 5000 credits
• Re-auth same user → balance unchanged

🤖 Generated with Claude Code