fix(edit): re-validate path immediately before write (TOCTOU hardening)

edit_apply validated the target with IsPathAllowed in the handler, then later
wrote it via os.WriteFile on the raw path after reading+parsing+diffing. A
symlink swapped in during that window could redirect the write outside the
workspace root. Inject the admission check into the edit Engine
(NewEngineWithValidator) and re-run it inside a new writeValidatedFile helper
immediately before the write — IsPathAllowed resolves symlinks at write time, so
a swapped-in link now resolving outside the root is rejected. This narrows the
TOCTOU window to near-zero while still permitting legitimate in-workspace symlink
edits (unlike a blanket O_NOFOLLOW). writeValidatedFile also dedups the
perm-preserving write shared by AST and text modes.

Tests: rejected-path edit leaves the file untouched; file mode is preserved.

internal/server/server.go

@@ -54,7 +54,7 @@ func New(cfg *config.Config, logger *slog.Logger) (*Server, error) {
 		logger:   logger,
 		parser:   parserRegistry,
 		matcher:  query.NewMatcher(parserRegistry),
-		editor:   edit.NewEngine(parserRegistry),
+		editor:   edit.NewEngineWithValidator(parserRegistry, cfg.IsPathAllowed),
 		readSem:  make(chan struct{}, MaxConcurrentReads),
 		querySem: make(chan struct{}, MaxConcurrentQueries),
 	}