feat(vault): encrypted-cache persistence + offline unlock #29

Job	Run time
rustfmt	13s
clippy	21s
cargo-audit	3m 27s
test (macos-latest)	44s
test (ubuntu-latest)	29s
cargo-deny	38s
headless build (no TUI / clipboard deps)	28s
vault --version emits §13.2 attribution	30s
	6m 50s

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