feat(vault): token persistence + refresh #36

Job	Run time
clippy	21s
test (macos-latest)	33s
cargo-audit	3m 18s
headless build (no TUI / clipboard deps)	24s
cargo-deny	39s
vault --version emits §13.2 attribution	34s
test (ubuntu-latest)	33s
rustfmt	15s
	6m 37s

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