feat(vault-agent): scheduled background sync #45

Job	Run time
cargo-audit	3m 3s
test (macos-latest)	1m 15s
cargo-deny	39s
headless build (no TUI / clipboard deps)	31s
clippy	23s
vault --version emits §13.2 attribution	30s
test (ubuntu-latest)	32s
rustfmt	15s
	7m 8s

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