feat: add issue discovery share config

[CaptainTimon]

Summary

• Add issue_discovery_share to RepositoryConfig with the required default of 0.5
• Parse and validate issue_discovery_share from master_repositories.json as a finite float in [0, 1]
• Configure entrius/oc-1 with issue_discovery_share: 0.0 because that optimization track should not allocate rewards to issue discovery
• Add loader and live-config tests for defaults, explicit parsing, bounds rejection, and the oc-1 opt-out

Related Issues

Part of #1215. Covers required deliverable 8 only.

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• Tests added/updated
• Manually tested

uv run pytest tests/validator/test_load_weights.py -q
uv run ruff check gittensor/validator/utils/load_weights.py tests/validator/test_load_weights.py
uv run ruff format --check gittensor/validator/utils/load_weights.py tests/validator/test_load_weights.py
uv run pytest -q

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)

Note for reviewers: this PR intentionally adds only the config surface. The allocation/spill behavior that consumes issue_discovery_share is left to separate #1215 deliverable PRs.

[anderdc]

Fold these changes into a single PR addressing #1215, not 11 slices. Closing.