feat: Convert per-repo weight from unbounded multiplier to emission_share

[jonathanchang31]

Summary

This update changes repository allocation from per-PR weight multiplier to per-repo emission_share pool slicing.

• RepositoryConfig.weight → RepositoryConfig.emission_share ([0,1])
• Added RepositoryConfig.issue_discovery_share ([0,1], default 0.5)
• Enforced registry invariant: sum(emission_share) <= 1.0
• Removed repo-weight factor from per-PR/per-issue multiplier chains
• Moved allocation control to round aggregation (allocate-then-distribute)
• Recycle now comes from:
  • unclaimed repo slices (repo inactive on both PR + issue sides)
  • registry slack (1 - sum(emission_share))
• Top-level emissions collapsed to:
  • OSS_EMISSION_SHARE = 0.90
  • ISSUES_TREASURY_EMISSION_SHARE = 0.10
  • removed fixed RECYCLE_EMISSION_SHARE baseline

Related Issue

Closes: #1215

Change Type (select all)

• Feature
• Behavior change
• Refactor
• Config schema change
• Validation/invariant hardening
• Monetary policy update
• Test coverage update
• Breaking API change (external)
• Security fix
• Performance optimization only

Real Behavior Proof

Use these expected outcomes as acceptance checks:

1. All repos active, sum(emission_share)=1.0

   • OSS scoring: 90%
   • Treasury: 10%
   • Recycle: 0%

2. No activity on any repo, sum(emission_share)=1.0

   • OSS scoring: 0%
   • Treasury: 10%
   • Recycle: 90%

3. All repos active, sum(emission_share)=0.8

   • OSS scoring: 72% (0.9 * 0.8)
   • Treasury: 10%
   • Recycle: 18% (registry slack)

4. Repo with issue_discovery_share=0.3, PR active, issue inactive

   • Full repo slice stays in repo and spills to PR side
   • No recycle for that repo

5. Repo has no eligible nonzero scorers on both sides

   • Entire repo slice goes to recycle in same round
   • No rollover, no redistribution to other repos

[anderdc]

Issue #1215 deliverables missed: D7 (SQL schema changes ship without a migration script), D11 (within-repo spill tested in only one direction; spec says 'vice versa'), D6 (no issue_discovery_share=0 short-circuit test). Closing.