NER-381: Split remaining oversized sync test exception

[freezscholte]

Summary

• split crates/forge-cli/tests/forge_sync.rs by scenario group, moving later peer/divergence/conflict scenarios into forge_sync_peer.rs
• move shared sync integration helpers into tests/support/sync.rs
• remove the old forge_sync.rs line-count allowlist entry
• document forge-content-native/src/lib.rs as the single ADR-backed line-count exception with a no-growth cap

File sizes

• crates/forge-store/src/lib.rs: 191 lines
• crates/forge-cli/src/main.rs: 107 lines
• crates/forge-cli/tests/forge_sync.rs: 2,266 lines
• crates/forge-cli/tests/forge_sync_peer.rs: 1,679 lines
• crates/forge-cli/tests/support/sync.rs: 761 lines
• crates/forge-content-native/src/lib.rs: 4,730 lines, justified and capped

Verification

• rtk cargo fmt --all --check
• scripts/check-rust-line-count.sh
• rtk cargo test -p forge-cli (325 tests / 36 suites)
• rtk cargo test --workspace (603 tests / 62 suites)
• rtk cargo clippy --workspace --all-targets -- -D warnings
• rtk bash scripts/ci.sh (PASS=95 FAIL=0)

Linear: NER-381

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