feat(vault-agent): anti-leak hardening — disable core dumps + ptrace #75

Job	Run time
cargo-audit	3m 13s
vault --version emits §13.2 attribution	43s
headless build (no TUI / clipboard deps)	42s
rustfmt	15s
test (macos-latest)	54s
clippy	27s
test (ubuntu-latest)	46s
cargo-deny	39s
	7m 39s

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