feat: add evidence authority ledger template

[SproutSeeds]

Summary

Add a template-first evidence-authority ledger for post-read source-weight tracking.

Why

GPD already has plan-time reference roles and strong comparison/contract result scaffolding. What it still does not expose cleanly is a small machine-visible surface for how much authority a reviewed source should carry downstream.

Scope

• add src/gpd/specs/templates/evidence-authority-ledger.md
• add a nearby bibliography cross-reference
• refresh repo-graph artifacts and changelog
• keep the first pass docs/template-first

Validation

• uv run python -m scripts.sync_repo_graph_contract
• uv run pytest tests/test_metadata_consistency.py -v
• uv run pytest tests/test_release_consistency.py -v
• uv run pytest tests/core/test_repo_interdependency_graph.py -v
• uv run pytest tests/core/test_prompt_wiring.py -k repo_graph -v

Notes

This is intentionally a narrow strawman. It does not add CLI, runtime wiring, or validator code in the first pass.