Speed up the internal/cli lock and two-vault tests (closes #80)

The test that each changing command waits for the state directory lock
slept a fixed 100 ms per command. It now polls the goroutine stacks until
the command is parked in vault.LockStateDir, checks the state directory
is unchanged, and releases the lock; a command that takes no lock still
fails by finishing first.

newTwoVaultFs creates its two vaults, each with a passphrase unlocker,
once, and returns a fresh copy of them on every call, so the six path and
move tests no longer each pay for two passphrase key derivations.

Model: opus-5-5

TODO.md

@@ -25,6 +25,13 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-04: The `internal/cli` tests are back to about their time
+  before the state directory lock
+  (https://git.eeqj.de/sneak/secret/issues/80). The test that each
+  changing command waits for the lock releases it as soon as it sees the
+  command waiting there, instead of after a fixed 100 ms. The two vaults
+  with passphrase unlockers that the path and move tests start from are
+  made once and copied for each test.
 - 2026-10-03: `secret mv` rejects a move whose destination is the
   source (`mv --force x x`, `mv --force work:x work:`, or an empty
   destination, which defaults to the source name) before changing