refactor(store): move evidence proposals domains into modules

[freezscholte]

Summary

• Move evidence capture and summary DTOs plus record_evidence into crates/forge-store/src/evidence.rs.
• Move proposal creation, checks, decisions, review/readiness metadata, PR body rendering, proposal lookup, and evidence-backed policy evaluation helpers into crates/forge-store/src/proposals.rs.
• Preserve existing public API paths through lib.rs re-exports and keep shared/internal helpers crate-private.
• Lower the store lib.rs line-count cap from 7,982 to 5,962 and update ADR-0001 with the NER-377 audit note.

Verification

• rtk cargo check -p forge-store
• scripts/check-rust-line-count.sh
• rtk cargo test -p forge-cli --test forge_accept_export
• rtk cargo test -p forge-cli --test forge_tamper
• rtk cargo test -p forge-cli --test forge_review_surface
• rtk cargo test -p forge-cli --test forge_compare
• rtk cargo test -p forge-cli --test forge_propose_check
• rtk cargo test -p forge-cli --test forge_run_evidence
• rtk cargo test -p forge-cli --test forge_start_save
• rtk cargo fmt --all --check
• rtk cargo test --workspace — 603 passed across 61 suites
• rtk cargo clippy --workspace --all-targets -- -D warnings
• rtk bash scripts/ci.sh — PASS=95 FAIL=0

Fixes NER-377.
Related NER-371.

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