Stop an untouched secret field from deleting the stored credential

Making `null` actually clear a secret — which it had to, since a blanked
token was previously never revoked — turned the config editors into a
credential shredder. Secrets are `#[serde(skip_serializing)]`, so the
inputs always render empty whether or not one is stored, and the blur
handlers sent `value || null` unconditionally. Focusing the Git token
field and tabbing away deleted it, with nothing shown and no undo.

`useSecretField` encodes the rule: only a field the user typed in may
speak about a secret. Untouched, `patch()` contributes no key at all, and
Rust already distinguishes an absent key from an explicit null.

`withoutUntouchedSecrets` covers the structural half. `saveBedrock`
spreads `{ ...bedrock, ...patch }`, and when that falls back to
DEFAULT_BEDROCK_CONFIG the literal spells every secret out as `null` — so
editing the AWS region would have wiped the credentials as a side effect.

Also here: `WorkspaceSection` no longer saves a half-filled folder row,
which `update_project`'s new validation would refuse on every keystroke
between the two inputs; `snapshots_skipped` is declared on the wire type
rather than widened locally; and `#[must_use]` on `ProjectGuard` and
`ScrubOutcome` — which immediately caught the migration path discarding
its scrub outcome, the one scrub whose silence is expensive because the
layer it declined to clean is about to be committed.

The capability test reads the real file and is mutation-verified: adding
`core:default` back makes it fail. That grant pulls in an unscoped
`std::fs::read` of any host path and went unnoticed for months, because
nothing in the suite read the file at all.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GBq2rGum6GX7xXgsas1fDc

app/src-tauri/src/project_lock.rs

@@ -151,6 +151,10 @@ fn holders() -> &'static Mutex<HashMap<String, ProjectOp>> {
 /// predecessor already learned: a plain release statement is skipped by an
 /// early `?`, by a panic, and by the future simply being dropped. A guard is
 /// not.
+/// Dropping this releases the claim, so a caller that discards it has taken no
+/// lock at all — `let _ = try_acquire(...)` drops immediately and reads as
+/// success. `#[must_use]` makes that a compile warning rather than a race.
+#[must_use = "the claim is released as soon as this guard is dropped; bind it for the whole operation"]
 #[derive(Debug)]
 pub struct ProjectGuard {
     project_id: String,
@@ -245,8 +249,11 @@ mod tests {
         assert!(err.contains("snapshot is being compacted"), "{}", err);
         assert!(err.contains("starting or recreating"), "{}", err);
         drop(first);
-        // And it has to be retakeable the moment the holder goes away.
-        try_acquire(&p, ProjectOp::Recreate).expect("released on drop");
+        // And it has to be retakeable the moment the holder goes away. Bound
+        // rather than discarded: `#[must_use]` is what stops a real caller
+        // writing `try_acquire(...)` and believing it holds something.
+        let retaken = try_acquire(&p, ProjectOp::Recreate).expect("released on drop");
+        drop(retaken);
     }
 
     #[test]