feat: Add durable action evidence in once.sqlite

Summary

• Adds an RFC framing Once as a project graph plus project memory, with graph, runs, evidence, views, and loops as the public model.
• Adds a SeaORM-backed .once/once.sqlite workspace store with migrations and an evidence_records table.
• Records durable action evidence from exec, legacy run, and graph build/run/test, then exposes it through once query evidence.
• Documents the new query surface and covers the once exec evidence path in shellspec.

Testing

• mise exec -- cargo check -p once-cli
• mise exec -- cargo test -p once-core evidence
• mise exec -- cargo test -p once-cli query
• mise exec -- cargo clippy -p once-core -p once-cli --all-targets -- -D warnings
• mise exec -- cargo build --release -p once-cli
• mise exec -- shellspec spec/exec_spec.sh
• mise exec -- cargo fmt --all -- --check
• git diff --check

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