Add contribution profile and opportunity scores (GW-045)

[trippyogi]

Summary

• Add a generic contribution_profile (mode weights, lexical domains, platforms, WIP limits, stale_pr_days) with no Hermes-specific defaults.
• Add opportunity scoring primitives: documented benefit/effort formula, hard-constraint BUILD=0, platform execution_fit (does not change confidence).
• Hunt rank is unchanged. Config files without contribution_profile still parse.

Depends on #86. Does not modify CLI/MCP or contracts/index.ts / core/index.ts.

Test plan

• pnpm check
• pnpm eval:frozen (31/31)
• Bugbot on branch changes; remaining notes are pre-existing GW-043 policy items already handled on Attach issue routing to worth_check (GW-044) #87