C1 (critical): setup wrote the master password + API client_secret as
`vault kv patch key=value` argv, leaking them via /proc/<pid>/cmdline to
same-UID processes. Now written via stdin (key=- form); only email +
client_id (non-credentials) remain in argv.
I1: `get --json` refused on a TTY (was dumping the secret to scrollback).
M1: vaultLock now holds the per-user flock (it mutates bw state).
M4: bw login-detection parses status JSON instead of substring matching.
M5: clipboard path refuses when stderr is not a TTY (was silently failing).
M6: realRunner trims only trailing newline, preserving secret whitespace;
secret prompts likewise.
Adds security-property tests: no secret in argv across the get flow,
clipboard decision matrix, --json TTY gate, bw status parsing.
